Physical colored inks and coatings

ABSTRACT

A coating, such as ink or paint, is used, where particles in the coating are selected based on electric, magnetic, or light/photo properties, and are dispersed in the coating to provide a desired physical color. In one approach, the application of the coating to the substrate such as paper is controlled using an electric or magnetic field. In another approach, a pattern is formed in a coating on a substrate by targeting an electric, magnetic or photo field to specific locations on the coating. In yet another approach, the color of a coating that is applied to an object is shifted to match a background color so that the coating appears to be erased. In this approach, the coating may be in the form of a label, such as a bar code, that can be read by a scanner at a point of sale location. In another approach a pattern or code is scrambled or removed by applying an electric, magnetic, or photo field to specific locations on the coating or substrate.

BACKGROUND OF THE INVENTION

Physical coloring, also referred to as coloration using particlescattering, provides many benefits relative to previous colorationapproaches such as dyes and pigments. The previous approaches sufferproblems with fading, recyclability, toxicity and other environmentalconcerns. Moreover, physical coloring enables many new properties, suchas switchability from one color state to another due to light exposure,temperature changes, or humidity changes. Various compositions thatachieve physical coloring are discussed in commonly-assigned U.S. Pat.No. 5,932,309, entitled “Colored Articles And Compositions And MethodsFor Their Fabrication”, issued Aug. 3, 1999, and incorporated herein byreference.

In particular, physical coloration can provide switchability from onecolor state to another. Such color changing compositions can be used,for example, for cosmetic purposes in polymer fibers used for textilesand carpets, and for color-changing windows and displays. Additionally,this type of technology could be used in military applications forcamouflage clothing, tents, and machinery. If such color change isreversibly switched as a consequence of light exposure, temperaturechanges, or humidity changes, then chameleon effects can be achieved forsuch articles. If the color switching effect is a one-time event causedby actinic radiation or high temperature exposure, the switching effectcan be used to provide spatially dependent coloration. Enhancing thevalue of polymer films, fibers, coatings, and other articles byachieving novel optical effects provides a major commercial goal.

In the prior art, U.S. Pat. No. 4,886,687 describes non-pigmentedcoloration as a result of diffraction effects originating from anembossed pattern having 5,000 to 100,000 lines per inch. U.S. Pat. No.4,017,318 describes glass articles that, after exposure to actinicradiations, can be heat treated to provide coloration effects because ofcolloidal silver particles. U.S. Pat. Nos. 2,515,936; 2,515,943 and2,651,145 also describe methods of generating colored silicate glassesusing combinations of various colloidal metals, including colloidal goldand silver. Pearlescent compositions, such as described in U.S. Pat.Nos. 3,087,829 and 4,146,403, provide coloration due to the interferenceof light reflected from parallel opposite sides of platelets depositedon the plate sides of mica substrate particles. U.S. Pat. No. 3,586,417shows that the wavelength at which a Christiansen filter transmits canbe varied for an optical device by varying the temperature of thefilter. Such variation results from the different temperaturecoefficients for the refractive indices of the scattering particles andthe liquid matrix. Various new methods for producing Christiansenfilters, including some efforts to make solid-matrix optical devices,are described by Balasubramanian, Applied Optics 31, pp. 1574-1587(1992).

However, the prior technologies do not provide the advantages of usingcoloration associated with particle scattering, or materials and methodsfor modifying and enhancing the coloration effects of particlescattering. Moreover, the prior technologies do not provide inks andother coatings that are based on physical color technology, and whichcan be controlled based on their electrical, magnetic, and/or photoproperties. The present invention addresses these and other issues.

SUMMARY OF THE INVENTION

The present invention provides a processable ink or other coating whichis based on physical color technology, and an apparatus and method forcontrolling the application of the coating to a substrate, as well asthe appearance of the coating once applied. The invention allows theinks or other coatings to acquire new properties which enable their usein new applications.

In accordance with the present invention, there is provided an apparatusfor controlling an application of a coating to a substrate. Theapparatus includes a field generator for providing an electrical and/ormagnetic field, and a controller for controlling the field to control anapplication of the coating to the substrate. The coating includesparticles having desired electric and/or magnetic properties, and theparticles are dispersed in the coating to provide a desired physicalcoloration.

In another aspect of the invention, an apparatus is provided for forminga pattern in a coating that is applied to a substrate. The apparatusincludes a field generator for providing an electrical, magnetic, and/orphoto field, and a controller for controlling an application of thefield to the coating to form the pattern therein. The coating includesparticles having desired electric, magnetic and/or photo properties, andthe particles are dispersed in the coating to provide a desired physicalcoloration.

In yet another aspect of the invention, an apparatus is provided forcontrolling the coloration of a coating applied to a substrate as alabel. The apparatus includes a field generator for applying anelectric, magnetic and/or photo field to the coating to cause a desiredchange in a physical coloration of the label. The coating includesparticles having desired electric, magnetic, and/or photo properties,and the particles are dispersed in the coating to provide a desiredphysical coloration. Moreover, the label is adapted to be read by ascanner such as a bar code scanner at a point of sale terminal in aretail store or for use in security and tracking purposes.

Another aspect of the invention provides a coating for application to asubstrate comprising fine particles that comprise particle scatteringcolorants, the particles having electric properties, magneticproperties, photo properties or a combination thereof; and a matrixcomponent, wherein application of an electric field, a magnetic field, aphoto field or a combination thereof to the fine particles causesagglomeration of the fine particles in a matrix and a concomitant colorshift, color reduction, color loss, or both a color shift and reductionin the coating.

Another aspect of the invention provides a method of producing a patternin a substrate or within a coating for application to a substratecomprising fine particles that function as particle scatteringcolorants, the particles having electric properties, magneticproperties, photo properties or a combination thereof; and a matrixcomponent, wherein application of an electric field, a magnetic field, aphoto field or a combination thereof to the fine particles causesagglomeration of the fine particles in a matrix and a concomitant colorshift, color reduction, color loss, or both a color shift and reductionin the coating resulting in a scrambling or an erasing of the pattern.

In another aspect, the invention provides a coating for application to asubstrate comprising fine particles that provide color via lightscattering, the particles having electric properties, magneticproperties, photo properties or a combination thereof, and a matrixcomponent wherein application of an electric field, a magnetic field ora photo field to an agglomeration of the fine particles reverses a colorreduction, color loss and/or a color shift associated with theagglomeration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus for controlling the application of acoating to a substrate in accordance with the present invention;

FIG. 2 illustrates an apparatus for changing the color of a coating thatis applied to an object as a label in accordance with the presentinvention; and

FIG. 3 illustrates an apparatus for selectively changing the color of acoating that is applied to a substrate to provide a desired pattern inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Physical color results from using unique particle distributions of fineparticles dispersed in a medium. The actual color of the material inbulk is of little concern since the cause of the color in the fineparticles results from light scattering. Hence, one can choose anyparticle independent of the desired final color. Moreover, one canchoose the particle based on other criteria, such as electrical,magnetic, and photo properties of the material. For instance, one canchoose a material based on conductive/electric properties or magneticproperties and make a fine dispersion for use in an ink formulation. Inthis case, the printing using the ink can be controlled by controllingthe fields surrounding the application. For example, a width of a sprayof the coating, and a direction of the spray can be controlled.Additionally, a pattern can be produced on a coating by selectivelyapplying a field according to a desired pattern, e.g., using technologysuch as a raster or electron gun. Or, one can control the properties ofthe material by developing a photo responsive dispersant or bound layer.Accordingly, the present invention provides inks or other coatings witha myriad of new properties.

Additionally, the invention leverages the shift, reduction, and/or lossof color associated with the agglomeration of fine particles. In oneembodiment of this invention, an externally applied stress or field isused to agglomerate the particles thereby causing a color shift,reduction, and/or loss and providing an erasable ink or coating.

Such an approach is of value in many applications, including inks orother coatings for plastics, metals and/or ceramics. Additionally, newpainting approaches can be provided based on these conductive inks ormagnetic inks. Erasable ink or photo changing inks/coatings can also beprovided. As an example, a bar code can be provided on a product, or alabel for the product, that essentially disappears by changing to thecolor of a background color of the product or label when it is scannedat checkout counter in a retail store, e.g., to obtain a price. Inanother example, a colored marker can be provided on a medicalinstrument or food package that is subject to sterilizing light or otherradiation, where the marker changes its color to confirm thesterilization has occurred.

The shift, reduction, and/or loss of color associated with theagglomeration of fine particles can also be leveraged in other ways. Inthis embodiment, an externally applied stress or field is used toagglomerate the particles thereby causing a color shift, reduction,and/or loss of the particles which results in a scrambling or an erasingof a code or pattern on the object when desired. Such an approach wouldbe valuable for security or tracking purposes.

The color shift, color reduction, or color loss described above can bewithin a substrate or within a coating on a substrate. In oneembodiment, the color shift, color reduction, and/or color loss iswithin a coating on a substrate. FIG. 1 illustrates an apparatus forcontrolling the application of a coating to a substrate in accordancewith the present invention. Here, a supply 110 of coating, e.g., ink orpaint, is communicated via an outlet 115 toward a substrate 120. Thecoating may be under pressure, such as in a paint sprayer, which causesit to be propelled from the outlet 115. The substrate 120 may includepaper or any other desired surface on which the coating is to beapplied. An electromagnetic field generator 130 generates an electric,magnetic, and or photo field to control the application of the coatingon the substrate 120. As is known, an electric field is produced byelectrically charged particles such as electrons. A magnetic field isgenerated when electric charge carriers such as electrons move throughspace or within an electrical conductor. An electromagnetic (EM) fieldis generated when charged particles are accelerated. Thus, the electricand/or magnetic field may be provided in any of the above contexts.Devices for providing such fields are known in the art, such as thosefrom New England TechniCoil, Inc. (Tuftonboro, N.H.) and Tabtronics,Inc. (Geneseo, N.Y.).

As indicated above, the coating may be any ink, or paint or othercoating material known in the art. The particle scattering colorants inthe coating are selected based on their electric, magnetic and/or photoproperties so that their trajectory toward the substrate 120 can becontrolled by adjusting the strength and/or position of the field. Forexample, the particle scattering colorants may be gold or silver, whichhave a high conductivity. Moreover, the particles in the coating aredispersed to provide a desired physical coloration to the coating. Anx-y controller 140 provides control signals to the field generator 130to direct the coating to a desired location on the substrate 120 whichcan be defined by any coordinate system, including Cartesian coordinatesand polar coordinates.

Additionally, as defined below, particle scattering colorants mayconsist of at least two layers also known as the onion-skin particlemethod. In this embodiment, a particle scattering colorant can compriseat least one layer comprising a particle scattering colorant and atleast one layer comprising a material selected based on its electric,magnetic, and/or photo properties so that their trajectory toward thesubstrate 120 can be controlled by adjusting the strength and/orposition of the field.

FIG. 2 illustrates an apparatus for changing the color of a coating inaccordance with the present invention. Here, the coating has beenapplied as a label on an object 210 either directly, or on a labelsubstrate 215 that is affixed to the object. Such a label substrate mayhave an adhesive backing, for example, for this purpose. The object 210may be a product (e.g., goods or service) that is purchased by aconsumer at a point of sale terminal in a retail store. Or, the object210 may be an item that is tracked or inventoried, e.g., on an assemblyline or in a warehouse. The coating may provide a label message such asprovided by bar codes which are commonly used to provide a price and/oran identifier associated with an object, or to designate a location atwhich the object should be stored or moved to, e.g., in a warehouse. Thecoating provides a physical coloration that allows it to be read by thebar code scanner 230 in a conventional manner. The physical colorationcan be achieved by dispersing fine particles in the coating as discussedherein. For example, a dark color on a white or other light coloredbackground can be read by the bar code scanner 230. A checkout register240 communicates with the bar code scanner 230 to add the price ofmultiple objects that the consumer is purchasing.

Moreover, the particle scattering colorants can be selected based onelectrical/conductive, magnetic, and/or photo properties so that thecolor can be shifted when an appropriate field is applied, such as bythe color field erase device 220. When the particles are selected basedon their electrical properties, an electric field is used by the colorfield erase device 220 to shift the color, reduce the color intensityand/or to scramble the pattern within the coating. When the particlesare selected based on their magnetic properties, a magnetic field isused by the color field erase device 220 to shift the color, reduce thecolor intensity, and/or to scramble a pattern within a coating. When theparticles are selected based on their photo properties (e.g., theirresponse to light), a photo/light field is used by the color field erasedevice 220 to shift the color, to reduce the color intensity and/or toscramble a pattern within a coating. In this case the particles orcoating may comprise a photo responsive material such as a dispersant orbound layer. Accordingly, the color field erase device 220 may compriseany device, capable of shifting electrical/conductive, magnetic and/orphoto properties.

For example, the color of the coating that forms the label message canbe shifted from a color that contrasts with the object or labelsubstrate to a color that essentially blends in by applying anelectrical/conductive and/or magnetic field to the coating so that theparticles agglomerate. In this manner, the coating essentially appearsto be erased on the object 210′. The color erase field device 220 maycommunicate with the bar code scanner 230 so the color is not changeduntil it is confirmed that the bar code has been successfully scanned.

In one possible embodiment, the label substrate 215 (or object 210, ifthe coating is applied directly to the object) may comprise acomposition having a solid matrix component with an electronictransition colorant, dye or pigment dispersed in it, all of which arereadily available to a skilled artisan. This composition therefore has afixed color that is generally unaffected by any applied field. Thecoating, which is disposed on the label substrate (or object), maycomprise a composition having a solid matrix component with a particlescattering colorant dispersed in it. The colorant imbues the coatingwith a color which can contrast with the color of the substrate 215 orobject 210. The colorant may comprise gold or silver, for example whichare affected by an applied electric or magnetic field. When such a fieldis applied by the color erase field device 220, the coloration due tothe particle scattering is attenuated such that the composition of thecoating becomes substantially transmissive to visible light reflectingfrom the substrate composition, and the coating essentially disappears.As defined below, colorants are considered particle scattering colorantsfor the purpose of this invention only if coloration depends on the sizeof the particles and there is no significant coloration from theinterference of light reflected from opposite sides or interfaces ofparallel plates. By definition nonabsorbing particle scatteringcolorants do not absorb visible light. When these particles agglomeratethe color caused by scattering is reduced or eliminated leaving a whiteor substantially transparent coating depending upon concentration of theagglomerated particles. By definition absorbing particle scatteringcolorants absorb visible light, however in this invention thesecolorants are used at very low concentrations (for example, 0.05% byweight for Au). When these absorbing particle scattering colorants beginto agglomerate their color shifts according to the Mie theory. When theparticles agglomerate to a large enough size, they no longer undergosubstantial Mie scattering. At these low concentrations, the observedcolor is reduced, substantially reduced or in some cases disappears.

In one embodiment the appearance of the object is improved when thecoating is erased or changed. In another embodiment it can be confirmedthat the object has been scanned. In yet another embodiment the originalcode or pattern on the object is scrambled or erased when desired forsecurity or tracking purposes.

The color erase field device 220 may further apply an appropriateelectric, magnetic and/or light field to the coating after it has beenerased to reverse the color change via de-agglomeration of the fineparticles so that the original code or pattern is visible again.

The coating may be used in various other ways. For example, the coatingmay be used in an environment that already employs an electric,magnetic, and/or photo field for other purposes, such as for sterilizinginstruments. For instance, the coating may be provided as a marker on amedical or dental instrument or other object that is sterilized usingultraviolet light. Here, the color of the coating indicates whether thelight has been applied, and, consequently, whether the instrument hasbeen sterilized. The coating may similarly be applied as a marker on afood package that has been irradiated to kill bacteria to confirm, viacolor change, that the irradiation has been applied. The coating mayalso be used to indicate the degree of the applied field based on thechange of color.

FIG. 3 illustrates an apparatus for selectively changing the color of acoating that is applied to a substrate to provide a desired pattern inaccordance with the present invention. Here, a coating 225 is applieduniformly or otherwise to a substrate 320, and a field generator 330which is responsive to an x-y controller 340, selectively directs anelectric, magnetic, and/or photo field to the coating 325. For example,technology such as a raster or electron gun can be used to provide aelectric and/or magnetic field. A photo field can be provided by using asuitable light beam. Based on the positioning of the field, acorresponding pattern is formed in the coating 325. The pattern may be adesign, text, or logo, for example. Moreover, the field may be directedtoward the front side of the coating, or to the backside when a field isused that can pass through the substrate.

In another possible embodiment, the substrate 320 may comprise acomposition having a solid matrix component having an electronictransition colorant, dye or pigment dispersed in it. This composition325, which is disposed on the substrate 320, may comprise a compositionhaving a solid matrix component with a particle scattering colorantdispersed in it. The colorant imbues the coating 325 with a color whichcan contrast with the substrate 320. The colorant may comprise gold orsilver, for example, which are affected by an applied electric ormagnetic field. When such a field is applied by the field generator 330,the coloration due to the particle scattering is attenuated only in thespecific locations where the field is applied such that the compositionof the coating becomes transmissive to visible light reflecting from thesubstrate composition, and the color of the substrate can be viewed. Thedesired pattern can be formed in this manner.

The coating can be prepared by methods known by those skilled in the artand discussed for example in commonly-assigned U.S. Pat. No. 5,932,309,entitled “Colored Articles And Compositions And Methods For TheirFabrication”, issued Aug. 3, 1999. The coating can be prepared bydispersing a nonabsorbing particle scattering colorant as a concentrateinto the coating solution followed by a let-down to a lowerconcentration. Milling and dispersants can be helpful in improvingdispersion of the particle scattering colorants. Specifically anonabsorbing particle scattering colorant such as MT-500B (an uncoatedtitanium dioxide from Daicolor-Pope having an average particle diameterof 35 nm) is milled overnight in a dispersing agent such as Caplube™ (avegetable-based material) at a 40% by weight concentration. Thisconcentrate is then added to the coating solution to prepare a coatingsolution comprising a final let-down concentration of, for example, 1%of the particle scattering colorant. In another method the coating isprepared by adding the particle scattering colorant directly to thecoating, coating solution, or coating material without a dispersingagent.

In another embodiment, an absorbing particle scattering colorant isproduced in situ in the coating, coating solution, or coating material.This method is known for solutions and polymers in the art and isdiscussed in commonly-assigned U.S. Pat. No. 5,932,309. In one exampleof this embodiment a metal salt is added to the coating solution. Thismetal salt is reduced in the coating solution to produce an absorbingparticle scattering colorant within the coating solution. Specifically,a gold (III) chloride solution is dissolved in the coating solution at a0.05% by weight gold concentration. A reducing agent, such as trisodiumcitrate, dissolved in an aqueous solution is then added with rapidstirring to the coating solution producing particle scattering colorantsin situ. Heating the solution, to for example 100° C., can facilitatethe reduction.

In materials that exhibit physical coloring, light scattering isexperienced by particles that are dispersed within matrices which are atleast partially light transmissive.

The colorants useful for this invention are called particle scatteringcolorants. Such colorants are distinguished from colorants that providecoloration due to the interference between light reflected from oppositeparallel sides or interfaces of plate-like particles, called plate-likeinterference colorants, and those that provide coloration due toelectronic transitions, called electronic transition colorants. Whileparticle scattering colorants can provide a degree of coloration byelectronic transitions, a colorant is a particle-scattering colorant ifcoloration depends on the size of the particles and there is nosignificant coloration from the interference of light reflected fromopposite sides or interfaces of parallel plates.

Particle scattering colorants are either absorbing particle scatteringcolorants or non-absorbing particle scattering colorants depending onwhether or not the particle scattering colorants significantly absorblight in the visible region of the spectrum. Absorption is evidenced bythe visual perception of color when particle sizes are sufficientlylarge that particle scattering of light is not significant.

For a first category, a particle colorant is used by dispersing it in asolid matrix that has a substantially different refractive index in thevisible than that of the particle scattering colorant. For this firstcategory, a particle scattering colorant is defined as a material thathas either the A or B property as defined below.

The A or B properties are determined by dispersing the candidateparticle scattering colorant in a colorless isotropic liquid that has arefractive index that is as different from that of the candidateparticle scattering colorant as is conveniently obtainable. The mostreliable test will result from choosing the refractive index differenceof the liquid and the candidate particle scattering colorant to be aslarge as possible. This liquid-solid mixture containing only thecandidate particle scattering colorant and the colorless isotropicliquid is referred to as the particle test mixture. The negativelogarithmic ratio of transmitted light intensity to incident lightintensity (−log(I/I_(o))) is measured for the particle test mixture as acontinuous function of wavelength over a wavelength range that includesthe entire visible spectral region from 380 to 750 nm. Suchmeasurements, can be conveniently accomplished using an ordinaryUV-visible spectrometer. The obtained quantity (−log(I/I_(o)) is calledthe effective absorbance, since it includes the effects of bothscattering and absorption on reducing the intensity of transmittedlight.

The A property is only a valid determinant for particle scatteringcolorants for materials which do not significantly absorb in the visibleregion of the spectrum, which means that absorption is not so large asto overwhelm the coloration effects due to particle scattering. For thesole purpose of the A property test, a material that does notsignificantly absorb in the visible region is defined as one whoseparticle test mixture has an effective maximum absorbance in thespectral region of from about 380 to about 750 nm that decreases by atleast about 2 times and preferably at least about 3 times when theaverage particle size of the candidate particle scattering colorant isincreased to above about 20 microns without changing the gravimetricconcentration of the candidate particle scattering colorant in theparticle test mixture.

It should be understood that the above described ratios of absorbanceswill in general have a weak dependence on the concentration of thecandidate particle scattering colorant in the particle test mixture.Such dependence is usually so weak as to be unimportant for thedetermination of whether or not a material is a particle scatteringcolorant. However, for cases where a material is only marginally aparticle scattering colorant (or is marginally not a particle scatteringcolorant) the above described ratios of absorbances should be evaluatedat the concentration of the candidate particle scattering colorantintended for materials application. Also, it will be obvious to oneskilled in the art that the concentration of the candidate particlescattering colorant in the test mixture should be sufficiently high thatI/I_(o) deviates significantly from unity, but not so high that I is toosmall to reliably measure.

A particle scattering colorant candidate that does not significantlyabsorb in the visible has the A property if the particle test mixturehas an effective maximum absorbance in the spectral region of from about380 to about 750 nm that is at least about 2 times and preferably atleast about 3 times the effective minimum absorbance in the samewavelength range and the average particle size of the material is belowabout 20 microns.

If the candidate particle scattering colorant is significantly absorbingin the visible, it can alternatively be determined to be a particlescattering colorant if another material has the A property and thatmaterial does not significantly absorb in the visible and hassubstantially the same distribution of particle sizes and shapes as thecandidate particle scattering colorant.

For scattering colorant candidates that significantly absorb in thevisible, the B property is also suitable for determining whether or nota particulate material is a particle scattering colorant. Thedetermination of whether or not the B property criterion is satisfiedrequires the same measurement of effective absorbance spectra in thevisible region as used above. The B property criterion is satisfied ifthe candidate particle scattering colorant has a minimum in transmittedlight intensity that is shifted at least by 10 nm compared with thatobtained for the same composition having an average particle size above20 microns.

In another embodiment, a colorant is formed when small particles, calledprimary particles are embedded within large particles. For this case,one can determine whether or not the candidate material is a particlescattering colorant by applying either the A property criterion or the Bproperty criterion to either the primary particles or to the embeddingparticles that contain the primary particles.

These complexities in determining what is a particle scattering colorantdisappear for embodiments of the second category, wherein the refractiveindex of a particle scattering colorant is matched to that of the matrixmaterial at some wavelength in the visible. In such cases, any materialthat has a particle size less than 2000 microns is a particle scatteringcolorant. Likewise, the determination of whether or not a candidate is aparticle scattering colorant is readily apparent when it comprises atwo-dimensional or three-dimensional ordered array of primary particles.Large particles of such particle scattering colorants will have anopal-like iridescence that is apparent to the eye.

While the above determinations of whether or not a particulate materialis a particle scattering colorant might seem complicated, they are quitesimple and convenient to apply. Particulate materials are much easier todisperse in liquids than they would be to disperse in the solid matricesthat provide the articles for use with this invention. Also, themeasurements of effective absorbance required for applying either the Aor B property criterion are rapid and can be accomplished byconventionally applied procedures using an inexpensive spectrometer.Hence, the application of these property criteria saves a great deal oftime in the identification of materials (i.e., particle scatteringcolorants) that are suitable for the practice of this invention.

In certain embodiments, electronic transition colorants are used inconjunction with particle scattering colorants. An electronic transitioncolorant is defined as a material that has an absorption coefficientgreater than 10⁻¹ cm.⁻¹ at a wavelength in the visible and does notsatisfy the criteria for a particle scattering colorant. Dyes andpigments are also used in conjunction with particle scattering colorantsin embodiments of this invention. A dye or pigment is defined as amaterial that absorbs light in the visible to a sufficient extent toconfer visibly perceptible coloration. Depending on particle size, apigment can either be a particle scattering colorant or an electronictransition colorant. Also, in general, either electronic transitioncolorants, dyes, or pigments can be used interchangeably in inventionembodiments.

In use, the particle scattering colorants used in the present inventionare dispersed as particles in a surrounding matrix. These particlescattering colorants particles can be either randomly located orarranged in a positionally correlated manner within a host matrix. Ineither case, intense coloration effects can occur as a consequence ofscattering from these particles. A positionally correlated arrangementof particle scattering colorants is preferred in order to achievecoloration effects that are somewhat flashy, and in some cases providedramatically different coloration for different viewing angles. Suchscattering processes for arrays of particles that have translationalorder are referred to as Bragg scattering. Non-correlated particlescattering colorants are preferred in order to achieve more subtlecoloration effects, which can be intense even for non-absorbing particlescattering colorants.

Since the visual limits of light radiation are approximately between 380and 750 nm, these limits are preferred to define the opticalcharacteristics of the particle scattering colorants for the purposes ofthe present invention. In some embodiments of the invention, theparticle scattering colorants that are preferred have a refractive indexthat is different from that of the host matrix throughout the entirevisible spectral range from 380 to 750 nm and particle scatteringeffects are preferably enhanced using electronic transition colorants,dyes or pigments. This situation differs from that of the Christiansenfilter materials of the prior art that provide matching of therefractive indices of host and matrix materials at least at onewavelength in the visible, and electronic transition colorants, dyes orpigments usually degrade performance. Unless otherwise specified, thedescribed refractive indices are those measured at room temperature.Also, a particle scattering colorant is said to have a differentrefractive index, a lower refractive index, or a higher refractive indexthan a matrix material if there exists a light polarization directionfor which this is true.

The particle scattering colorants, or a subcomponent thereof, should besmall enough to effectively scatter light chromatically. If there doesnot exist a visible wavelength at which a refractive index of thescattering particle colorant and the matrix are substantially matched,this means that the average particle size of such colorants ispreferably less than about 2 microns in the smallest dimension. Byaverage particle size we mean the ordinary arithmetic average, ratherthan (for example) the root-mean-square average. For embodiments of thisinvention where chromatic coloration occurs as a consequence of theexistence of a large difference between the refractive index of thematrix and the particle scattering colorant throughout the visiblespectral region, the average particle size for the particle scatteringcolorants is more preferably from about 0.01 to about 0.4 microns. Inthis case the average particle size in the smallest dimension is mostpreferably less than about 0.2 microns. Especially if the particlescattering colorant significantly absorbs light in the visible, evensmaller average particle sizes of less than 0.01 microns are within thepreferred range. Also, if the particle scattering colorant particles arenot preferentially oriented, it is preferable that the average ratio ofmaximum dimension to minimum dimension for individual particles of theparticle scattering colorant is less than about four and that theparticle scattering colorant particles have little dispersion in eitherparticle size or shape. On the other hand, for embodiments of thisinvention in which the refractive index of the particle scatteringcolorant and the matrix substantially vanishes at a visible wavelength,particle shapes can be quite irregular and preferred average particlesizes can be quite large, preferably less than about 2000 microns. Evenlarger particle sizes can be in the preferred range if the particlescattering colorant contains smaller particle scattering colorantswithin it. This complicated issue of preferred particle sizes fordifferent embodiments of the invention will be further clarified in thediscussion of these embodiments hereinafter.

Instead of expressing particle sizes by an average particle size or anaverage particle size in the smallest dimension, particle size for aparticular particle scattering colorant can be expressed as the fractionof particles that have a smallest dimension that is smaller than adescribed limit. Such description is most useful for the embodiments ofthis invention where the refractive index of the particle scatteringcolorant is much different than that of the matrix at all wavelengths inthe visible. In such embodiments, it is preferable that at least about50% of all particles have a smallest dimension that is less than about0.1 microns.

The matrix in which the particle scattering colorant is dispersed can beeither absorbing or non-absorbing in the visible spectral range. Thisabsorption characteristic can be specified using eitherpath-length-dependent or path-length-independent quantities forcharacterization. For example, if an initial light intensity I_(o) isreduced to I_(t) by absorptive processes after the light passes througha matrix thickness t, then the percent transmission is 100(I_(t)/I_(o)).The corresponding absorption coefficient is −(1/t)ln((I_(t)/I_(o)).Unless otherwise specified, the described absorption characteristics arethose for a light polarization direction for which there is leastabsorption of light. For certain applications it is preferable for theparticle scattering colorant to be substantially non-absorbing in thevisible region. For other applications it is sufficient for the particlescattering colorant to not have a highest peak in absorption peak withinthe visible. In other applications that will be described, it ispreferable for the particle scattering colorant to have a maxima inabsorption coefficient at wavelengths that are within the visible. Thelatter provides invention embodiments in which the particle scatteringcolorant contains an overcoating layer of an absorbing material that issufficiently thin that it produces little light absorption.

Light scattering that is not strongly frequency dependent in the visibleregion will often occur as a result of imperfections in a matrixmaterial. One example of such imperfections are crystallite-amorphousboundaries in semi-crystalline polymeric matrix materials. Suchnon-chromatic scattering can interfere with the achievement ofcoloration using particle scattering colorants. Consequently, it isuseful to define the “effective absorption coefficient” using the aboveexpressions, without correction for the scattering of the matrix thatdoes not arise from the particle scattering colorants.

Because of their utility for the construction of various articles forwhich novel optical effects are desired, such as UPC codes, securitymarkings, carpets, clothing, wall paper, draperies, coverings forfurniture, polymer molded parts, and coatings, paper coatings andorganic polymers are two possible matrix materials for the compositionsof this invention. By polymers we mean homopolymers, copolymers, andvarious mixtures thereof. Various inorganic and mixed organic andinorganic matrix materials are also suitable for use as matrix materialsfor the present invention, such as SiO₂ glasses, and mixtures ofinorganic and organic polymers. The principal limitation on the choiceof such matrix materials is that either absorption or wavelengthinsensitive light scattering are not so dominant that thewavelength-selective scattering (i.e., chromatic scattering) due toparticle scattering colorants is negligible. This limitation means thatsuch matrix materials must have a degree of transparency. Using theabove defined effective absorption coefficient, this requirement oftransparency means that the effective absorption coefficient for thehost matrix in which the particle scattering colorant particles aredispersed is preferably less than about 10⁻⁴ Å⁻¹ at some wavelength inthe visible spectra. More preferably, this effective absorptioncoefficient of the host matrix is less than about 10⁻⁵ Å⁻¹ at somewavelength in the visible, and most preferably this effective absorptioncoefficient is less than about 10⁻⁶ Å⁻¹ at some wavelength in thevisible. Numerous commercially available transparent organic polymershaving lower effective absorption coefficients in the visible areespecially suitable for use as matrix materials for the presentinvention. These include, for example, polyamides, polyurethanes,polyesters, polyacrylonitriles, and hydrocarbon polymers such aspolyethylene and polypropylene. Amorphous polymers having very littlescattering due to imperfections are especially preferred, such as anoptical quality polyvinyl, acrylic, polysulfone, polycarbonate,polyarylate, or polystyrene.

Depending on the intensity of coloration desired, the loading level ofthe particle scattering colorant in the host matrix can be varied over avery wide range. As long as the particle scattering colorants do notbecome aggregated to the extent that large refractive index fluctuationsare eliminated at interfaces between particles, the intensity ofcoloration will generally increase with the loading level of theparticle scattering colorant. However, very high loading levels of theparticle scattering colorant can degrade mechanical properties andintimate particle aggregation can dramatically decrease interfacialrefractive index changes and alter the effective dimensions ofscattering particles. For this reason the volumetric loading level ofthe particle scattering colorant in the host matrix is preferably lessthat about 70%, more preferably less than about 30%, and most preferablyless than about 10%. However, in order to obtain a significantcoloration effect, the particle scattering colorant preferably comprisesat least about 0.01 weight percent of the matrix component. Also, therequired loading levels of particle scattering colorants can be lowerfor absorbing particle scattering colorants than for non-absorbingparticle scattering colorants, and can be decreased in certainembodiments of the invention as either the refractive index differencebetween matrix and particle scattering colorant is increased or thethickness of the matrix containing the particle scattering colorant isincreased.

Various methods of particle construction can be employed in thematerials of the present invention for achieving the refractive indexvariations that are necessary in order to obtain strong particlescattering. Preferred methods include (1) the simple particle method,(2) the surface-enhanced particle method, and (3) the onion-skinparticle method. In the simple particle method, the particles aresubstantially uniform in composition and the refractive index of theseparticles is chosen to be different from that of the host matrix. Unlessotherwise noted, comments made herein regarding the refractive indexdifferences of particles and host matrices pertain either to theparticle refractive index for the simple particle method or the outerparticle layer for the case of more complex particles. In thesurface-enhanced particle method, the particles contain an overcoat ofan agent that has a refractive index which is different from that of thematrix. The refractive indices of the surface enhancement agent and thehost matrix should preferably differ by at least about 5%. Morepreferably, this refractive index difference is greater than about 25%.Finally, in the onion-skin particle method, the scattering particles aremulti-layered (like an onion skin) with layers having differentrefractive indices, so that scattering occurs from each interfacebetween layers. This refractive index difference is preferably greaterthan about 5%, although smaller refractive index differences can beusefully employed if a large number of layers are present in theonion-skin structure.

In one embodiment of this invention for the simple particle method therefractive index of the scattering particles is higher than that of thematrix. In another embodiment the refractive index of the matrix ishigher than that of the scattering particles. In both these embodimentsthe difference in refractive indices of the scattering centers and thematrix should be maximized in order to enhance coloration due toparticle scattering. Hence, these embodiments are referred to as largeΔn embodiments. More specifically, in the case where the scatteringcenters are inorganic particles and the matrix is an organic polymer,the difference in refractive index between the inorganic particles andthe organic polymer should be maximized. This refractive indexdifference will generally depend on the direction of light polarization.

In other embodiments of this invention, the refractive index of theparticle scattering colorants are closely matched at least at onewavelength in the visible. In these embodiments it is preferred that (1)there is a large difference in the wavelength dependence of therefractive index of the particle scattering colorant and the matrixpolymer in the visual spectral region, (2) the matrix polymer and theparticle scattering colorant have states that are optically isotropic,and (3) the neat matrix polymer has a very high transparency in thevisible. Such embodiments, called vanishing Δn embodiments, use theconcept of the Christiansen filter to obtain coloration. The size of theparticle scattering colorants are chosen so that all wavelengths in thevisible region are scattered, except those in the vicinity of thewavelength at which the refractive index of the matrix and the particlescattering colorant are matched. This wavelength dependence ofscattering efficiency either provides or enhances the articlecoloration.

Both the high Δn embodiments and the vanishing Δn embodiments providethe means for obtaining either stable coloration or switchablecoloration. In the high Δn embodiments, coloration that is switchable ina desired manner is preferably achieved using the combined effects ofparticle scattering and a wavelength-dependent absorption in the visiblethat is associated with an electronic transition. In the vanishing Δnembodiments, coloration that is switchable in a desired manner can beachieved by effects (light or actinic radiation exposure, thermalexposure, electric fields, temperature, humidity, etc.) that either (1)shift the wavelength at which Δn vanishes between two wavelengths withinthe visible range, (2) shift the wavelength at which Δn vanishes towithin the visible range, (3) shift the wavelength at which Δn vanishesto outside the visible range, or (4) causes a shift in coloration due tocombined effects of particle scattering and chromism in absorption inthe visible that is associated with an electronic transition colorant,dye or pigment. Ferroelectric, switchable antiferroelectriccompositions, and photoferroelectric compositions provide preferredcompositions for obtaining switchable coloration using particlescattering colorants.

Electronic transition colorants, dyes or pigments are especiallypreferred for obtaining switchable coloration for the high Δnembodiments, even when such colorants do not undergo a switching ofelectron absorption coloration. The reason can be seen by considering amaterial (such as a polymer film) that is sufficiently thin thatparticles do not scatter all of the incident visible radiation. In thiscase of the high Δn embodiment, the difference in refractive index ofthe particle scattering colorants and the matrix is large over theentire visible spectral range (compared with the wavelength dependenceof Δn over this range). Hence, changes in the refractive indexdifference between particle scattering colorant and matrix increases theoverall intensity of scattered light, which is generally approximatelyexponentially proportional to (Δn)², but does not substantially changethe wavelength distribution of such scattered light. On the other hand,the chromatic reflection and absorption of an electronic transitionabsorption colorant can provide switchability in the chromatic nature ofscattered light, since the amount of incident light effected by theelectronic transition colorant, dye or pigment can depend upon theamount of light that is not scattered by the particle scatteringcolorant. As an example, one may think of the situation where thescattering effectiveness and thickness of a particle scattering colorantlayer is so great that substantially no light is transmitted through toa layer containing an electronic transition colorant. If the refractiveindex of the particle scattering colorant is then switched so that therefractive index of the particle scattering colorant becomes much closerto that of the matrix, then light can be substantially transmittedthrough the particle scattering colorant layer to the electronictransition colorant layer. Then a switchability in the refractive indexof the particle scattering colorant provides a switchability in thecoloration of the article. This situation is quite different from thecase of the vanishing Δn embodiment, where, even in the absence of anelectronic absorption, an article that is sufficiently thin that it doesnot completely scatter light can evidence a switchability in thechromatic nature of scattered light. This can be true as long as thereis a switchability in the wavelength in the visible at which Δn vanishesand Δn significantly depends upon wavelength in the visible. Thewavelength dependence of refractive index in the visible is usefullyprovided as either n_(F)−n_(C) or the Abbe number((n_(D)−1)/(n_(F)−n_(C))), where the subscripts F, D, and C indicate thevalues of the refractive index at 486.1, 589.3, 656.3 nm, respectively.For the purpose of obtaining enhanced coloration for the vanishing Δnembodiment, the difference in n_(F)−n_(C) for the particle scatteringcolorant and the matrix in which this colorant is dispersed ispreferably greater in absolute magnitude than about 0.001.

Particle scattering colorants and electronic transition colorants caneither be commingled together in the same matrix or mingled in separatematrices that are assembled so as to be either substantially mutuallyinterpenetrating or substantially mutually non-interpenetrating. Thelatter case, where the particle scattering colorant and the electronicscattering colorant are in separate matrices that are substantiallymutually non-interpenetrating, provides the more preferred embodimentsof this invention, since the total intensity of light scattered by theparticle scattering colorant can thereby be optimized. In this type ofembodiment, the matrix containing the particle scattering colorant ispreferably substantially exterior to that containing the electronictransition colorant on at least one side of a fashioned article. So thatthe effects of both a electronic transition colorant and a non-absorbingparticle scattering colorant can be perceived, the thickness of thematrix containing the particle scattering colorant should be such thatthere exists a wavelength of visible light where from about 10% to about90% light transmission occurs through the particle scattering colorantmatrix layer, so as to reach the electronic transition colorant matrixlayer. The preferred thickness of the electronic absorption colorantcontaining matrix layer that underlies the particle scattering colorantcontaining layer (t_(e)) depends upon the absorption coefficient of theelectronic transition colorant at the wavelength in the visible at whichthe maximum absorption occurs (λ_(m)), which is called α_(e), and thevolume fraction of the matrix that is the electronic transition colorant(V_(e)). Preferably, α_(e) t_(e) V_(e) is greater than 0.1, whichcorresponds to a 9.5% absorption at λ_(m). Likewise, for the embodimentswhere the particle scattering colorant and the electronic absorptioncolorant are commingled in the same phase, it is useful to defineanalogous quantities for the particle scattering colorant (which aredenoted by the subscripts s), the only difference being α_(s) for theparticle scattering colorant includes the effects of both lightabsorption and light scattering on reducing the amount of lighttransmitted through the material and α_(s) depends on particle size. Forthese embodiments α_(e) V_(e) and α_(s) V_(s) preferably differ by lessthan a factor of about ten, and more preferably by a factor of less thanabout three. Likewise, preferred embodiments can be expressed for thecase of where the particle scattering colorant and the electronictransition colorant are located in separate phases (with volumes v_(s)and v_(e), respectively) that are substantially mutuallyinterpenetrating. In this case, α_(e) v_(e) V_(e) and α_(s) v_(s) V_(s)preferably differ by less than about a factor of ten, and morepreferably by a factor of less than about three.

The variation in refractive indices with composition for organicpolymers is relatively small compared with the corresponding variationfor inorganic particles. Typical average values for various unorientedorganic polymers at 589 nm are as follows: polyolefins (1.47-1.52),polystyrenes (1.59-1.61), polyfluoro-olefins (1.35-1.42), non-aromaticnon-halogenated polyvinyls (1.45-1.52), polyacrylates (1.47-1.48),polymethacrylates (1.46-1.57), polydienes (1.51-1.56), polyoxides(1.45-1.51), polyamides (1.47-1.58), and polycarbonates (1.57-1.65).Especially preferred polymers for use as polymer host matrices are thosethat have little light scattering in the visible due to imperfections,such as polymers that are either amorphous or have crystallite sizesthat are much smaller than the wavelength of visible light. The latterpolymers can be obtained, for example, by rapid melt-quenching methods.

Preferred scattering particles for combination in composites withpolymers having such low refractive indices in high Δn embodiments arehigh refractive index materials such as: 1) metal oxides such astitanium dioxide, zinc oxide, silica, zirconium oxide, antimony trioxideand alumina; 2) carbon phases such as diamond (n about 2.42),Lonsdaleite, and diamond-like carbon; 3) other high refractive indexinorganics such as bismuth oxychloride (BiOCl), barium titanate (n_(o)between 2.543 and 2.339 and n_(e) between 2.644 and 2.392 forwavelengths between 420 and 670 nm), potassium lithium niobate (n_(o)between 2.326 and 2.208 and n_(e) between 2.197 and 2.112 forwavelengths between 532 and 1064 um), lithium niobate (n_(o) between2.304 and 2.124 and n_(e) between 2.414 and 2.202 for wavelengthsbetween 420 and 2000 nm), lithium tantalate (n_(o) between 2.242 and2.112 and n_(e) between 2.247 and 2.117 for wavelengths between 450 and1800 nm), proustite (n_(o) between 2.739 and 2.542 and n_(e) between3.019 and 2.765 for wavelengths between 633 and 1709 nm), zinc oxide(n_(o) between 2.106 and 1.923 and n_(e) between 2.123 and 1.937 forwavelengths between 450 and 1800 nm), alpha-zinc sulfide (n_(o) between2.705 and 2.285 and n_(e) between 2.709 and 2.288 for wavelengthsbetween 360 and 1400 nm), and beta-zinc sulfide (n_(o) between 2.471 and2.265 for wavelengths between 450 and 2000 nm). High refractive indexorganic phases are also preferred as particle scattering colorants foruse in low refractive index phases. An example of a high refractiveindex organic phase that can be used as a particle scattering colorantwith a low refractive index organic matrix phase (such as apolyfluoro-olefin) is a polycarbonate or a polystyrene. As isconventional, n_(o) and n_(e) in the above list of refractive indicesdenote the ordinary and extraordinary refractive indices, respectively,for crystals that are optically anisotropic. The n_(o) refractive indexis for light propagating down the principal axis, so there is no doublerefraction, and the n_(e) refractive index is for light having apolarization that is along the principal axis.

For the case where a high refractive index matrix is needed inconjunction with low index scattering particles, preferred particlescattering colorants are 1) low refractive index materials, such asfluorinated linear polymers, fluorinated carbon tubules, fluorinatedgraphite, and fluorinated fullerene phases, 2) low refractive indexparticles such as cavities filled with air or other gases, and 3) lowrefractive index inorganic materials such as either crystalline oramorphous MgF₂. Various inorganic glasses, such as silicate glasses, arepreferred for use as particle scattering colorants in many organicpolymer matrices for the vanishing Δn embodiments. The reason for thispreference is that such glasses are inexpensive and can be convenientlyformulated to match the refractive index of important, commerciallyavailable polymers at one wavelength in the visible. Also, thedispersion of refractive index for these glasses can be quite differentfrom that of the polymers, so that substantial coloration effects canappear in particle scattering. Inorganic glasses are also preferred foruse in high Δn embodiments, although it should be clear that the hostmatrix chosen for a high Δn embodiment for particular glass particlesmust have either a much higher or a much lower refractive index than thematrix chosen for a vanishing Δn embodiment for the same glassparticles. For example a glass having a refractive index of 1.592 wouldbe a suitable particle scattering colorant for polystyrene in thevanishing Δn embodiment, since polystyrene has about this refractiveindex. On the other hand, poly(heptafluorobutyl acrylate), withrefractive index of 1.367 could be used with the same glass particles ina high Δn embodiment. Relevant for constructing these colorant systems,note that the refractive indices of common glasses used in opticalinstruments range from about 1.46 to 1.96. For example, the refractiveindices of ordinary crown, borosilicate crown, barium flint, and lightbarium flint extend from 1.5171 to 1.5741 and the refractive indices ofthe heavy flint glasses extend up to about 1.9626. The values ofn_(F)−n_(C) for these glasses with refractive indices between 1.5171 and1.5741 range between 0.0082 and 0.0101. The corresponding range of theAbbe number is between 48.8 and 59.6. A refractive index that is on thelower end of the above range for commonly used optical glasses isobtained for fused quartz, and this material is also a preferredparticle scattering colorant. The refractive index for fused quartzranges from 1.4619 at 509 nm to 1.4564 at 656 nm.

Ferroelectric ceramics (such as the above mentioned barium titanate andsolid solutions of BaTiO₃ with either SrTiO₃, PbTiO₃, BaSnO₃, CaTiO₃, orBaZrO₃) are preferred compositions for the particle scattering colorantphase of the compositions of the present invention. The reason for thispreference is two-fold. First, very high refractive indices areobtainable for many such compositions. For high Δn embodiments, thesehigh refractive indices can dramatically enhance coloration via anenhancement in scattering due to the large refractive index differencewith respect to that of the matrix phase. Second, if matrix and hostphases are matched in refractive index at a particular wavelength in theabsence of an applied field (as for the vanishing Δn embodiments), anapplied electric field can change the wavelength at which this matchoccurs—thereby providing a switching of color state. Alternatively, aferroelectric phase that is an organic polymer can be selected to be thehost phase. If a particle phase is again selected to match therefractive index of the unpoled ferroelectric at a particularwavelength, the poling process can introduce an electrically switchedchange in coloration. Such matching of the refractive index of hostphase and particle scattering colorant can be one that exists only for aspecified direction of light polarization. However, it is most preferredthat the matrix material and the particle scattering colorant havelittle optical anisotropy, so that the match of refractive indices islargely independent of light polarization direction.

Ceramics that are relaxor ferroelectrics are preferred ferroelectricsfor use as particle scattering colorant phases. These relaxorferroelectrics have a highly diffuse transition between ferroelectricand paraelectric states. This transition is characterized by atemperature T_(m), which is the temperature of the frequency-dependentpeak in dielectric constant. As is conventional, we herein call T_(m)the Curie temperature (T_(c)) of a relaxor ferroelectric, even thoughsuch ferroelectrics do not have a single transition temperature from apurely ferroelectric state to a purely paraelectric state. Relaxorferroelectrics are preferred ferroelectrics for use as particlescattering colorants when electric-field-induced switching in colorationis desired, since such compositions can display very large field-inducedchanges in refractive indices. Since these field-induced refractiveindex changes generally decrease as particle diameters become small, theparticle dimensions should be selected to be as large as is consistentwith achieving desired coloration states.

Relaxor ferroelectrics that are preferred for the present invention havethe lead titanate type of structure (PbTiO₃) and disorder on either thePb-type of sites (called A sites) or the Ti-type of sites (called Bsites). Examples of such relaxor ferroelectrics having B sitecompositional disorder are Pb(Mg_(1/3) Nb_(2/3))O₃ (called PMN),Pb(Zn_(1/3) Nb_(2/3))O₃ (called PZN), Pb(Ni_(1/3) Nb_(2/3))O₃ (calledPNN), Pb(Sc_(1/2) Ta_(1/2))O₃, Pb(Sc _(1/2) Nb_(1/2))O₃ (called PSN),Pb(Fe_(1/2) Nb_(1/2))O₃ (called PFN), and Pb(Fe_(1/2) Ta_(1/2))O₃. Theseare of the form A(BF_(1/3) BG_(2/3))O₃ and A(BF_(1/2) BG_(1/2))O₃, whereBF and BG represent the atom types on the B sites. Further examples ofrelaxor ferroelectrics with B-site disorder are solid solutions of theabove compositions, such as (1−x)Pb(Mg_(1/3) Nb_(2/3))O₃−xPbTiO₃ and(1−x)Pb(Zn_(1/3) Nb_(2/3))O₃−xPbTiO₃. Another more complicated relaxorferroelectric that is preferred for the present invention is Pb_(1−x) ²⁺La_(x) ³⁺ (Zr_(y) Ti_(z))_(1−x/4)O₃, which is called PLZT. PZT (leadzirconate titanate, PbZr_(1−x) Ti_(x) O₃) is an especially preferredferroelectric ceramic for use as a particle scattering colorant. PMN(lead magnesium niobate, Pb(Mg_(1/3) Nb_(2/3))O₃) is another especiallypreferred material, which becomes ferroelectric below room temperature.Ceramic compositions obtained by the addition of up to 35 mole percentPbTiO₃ (PT) to PMN are also especially preferred for use as a particlescattering colorant, since the addition of PT to PMN provides a methodfor varying properties (such as increasing the Curie transitiontemperature and varying the refractive indices) and since a relaxorferroelectric state is obtainable using up to 35 mole percent of added(i.e., alloyed) PT.

Ceramic compositions that undergo a field-induced phase transition fromthe antiferroelectric to the ferroelectric state are also preferred forobtaining composites that undergo electric-field-induced switching ofcoloration. One preferred family is the Pb_(0.97) La_(0.02) (Zr, Ti,Sn)O₃ family that has been found by Brooks et al. (Journal of AppliedPhysics 75, pp. 1699-1704 (1994)) to undergo the antiferroelectric toferroelectric transition at fields as low as 0.027 MV/cm. Another familyof such compositions is lead zirconate-based antiferroelectrics thathave been described by Oh et al. in “Piezoelectricity in theField-Induced Ferroelectric Phase of Lead Zirconate-BasedAntiferroelectrics”, J. American Ceramics Society 75, pp. 795-799 (1992)and by Furuta et al. in “Shape Memory Ceramics and Their Applications toLatching Relays”, Sensors and Materials 3,4, pp. 205-215 (1992).Examples of known compositions of this type, referred to as the PNZSTfamily, are of the general form Pb_(0.99) Nb_(0.02) [(Zr_(0.6)Sn_(0.4))_(1−y) Ti_(y)]_(0.98) O₃. Compositions included within thisfamily display field-induced ferroelectric behavior that is maintainedeven after the poling field is removed. Such behavior is not observedfor Type I material (y=0.060), where the ferroelectric state reconvertsto the antiferroelectric state when the field is removed. However, typeII material (y=0.63) maintains the ferroelectric state until a smallreverse field is applied and the type III material (y=065) does notrevert to the antiferroelectric state until thermally annealed at above50° C. Reflecting these property differences, the type I material can beused for articles that change coloration when an electric field isapplied, and revert to the initial color state when this field isremoved. On the other hand, the type II and type III materials can beused to provide materials in which the electric-field-switched colorstate is stable until either a field in the reverse direction is appliedor the material is thermally annealed.

Ferroelectric polymer compositions are suitable for providing either theparticle scattering colorant or the matrix material for a composite thatis electrically switchable from one color state to another. The termferroelectric polymer as used herein includes both homopolymers and allcategories of copolymers, such as random copolymers and various types ofblock copolymers. This term also includes various physical and chemicalmixtures of polymers. Poly(vinylidene fluoride) copolymers, such aspoly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), are preferredferroelectric polymer compositions. Additional copolymers of vinylidenefluoride that are useful for the composites of the present invention aredescribed by Tournut in Macromolecular Symposium 82, pp. 99-109 (1994).Other preferred ferroelectric polymer compositions are the copolymers ofvinylidene cyanide and vinyl acetate (especially the equal mole ratiocopolymer) and odd nylons, such as nylon 11, nylon 9, nylon 7, nylon 5,nylon 3 and copolymers thereof.

Other particle scattering colorants include those that are absorbingparticle scattering colorants. One preferred family of such absorbingparticle scattering colorants are colloidal particles of metals (such asgold, silver, platinum, palladium, lead, copper, tin, zinc, nickel,aluminum, iron, rhodium, osmium, iridium, and alloys, metal oxides suchas copper oxide, and metal salts). Preferably the particles are lessthan about 0.5 micron in average dimension. More preferably theparticles are less than about 0.1 microns in average dimension. In orderto achieve special coloration effects, particles are most preferred thatare less than about 0.02 microns in average dimension. Particles thathave colloid-like dimensions are herein referred to as colloidalparticles, whether or not colloid solutions can be formed. Particlesizes that are below about 0.02 microns are especially useful forobtaining a wide range of coloration effects from one composition ofabsorbing particle scattering colorant, since these small sizes canprovide particle refractive indices and absorption coefficient maximathat depend upon particle size. This size variation of the wavelengthdependent refractive index and absorption coefficient is most stronglyenhanced for particles that are sometimes referred to as quantum dots.Such quantum dot particles preferably have a narrow particle sizedistribution and an average particle size that is from about 0.002 toabout 0.010 microns.

Convenient methods for forming colloidal particles include the variousmethods well known in the art, such as reaction of a metal salt in asolution or the crystallization of materials in confined spaces, such assolid matrices or vesicles. Likewise, well-known methods for producingcolloidal particles can be employed wherein colloid size liquid or solidparticles dispersed in a gas or a vacuum are either reacted or otherwisetransformed into solid particles of desired composition, such as bycrystallization. As an example of formation of colloidal particles thatare useful for the present invention by solution reaction methods, notethat Q. Yitai et al. have described (in Materials Research Bulletin 30,pp. 601-605 (1995)) the production of 0.006 micron diameter zinc sulfideparticles having a very narrow particle distribution by the hydrothermaltreatment of mixed sodium sulfide and zinc acetate solutions. Also, D.Daichuan et al. have reported (in Materials Research Bulletin 30, pp.537-541 (1995)) the production of uniform dimension colloidal particlesof beta.-FeO(OH) by the hydrolysis of ferric salts in the presence ofurea using microwave heating. These particles had a rod-like shape and anarrow size distribution. Using a similar method (that is described inMaterials Research Bulletin 30, pp. 531-535 (1995)), these authors havemade colloidal particles of .alpha.−FeO having a uniform shape (anddimensions) that can be varied from a tetragonal shape to close tospherical (with an average particle diameter of about 0.075 microns). T.Smith et al. report the production of colloidal particles incommonly-assigned U.S. Pat. No. 5,932,309, wherein the colloidalparticles are prepared in situ by adding a metal salt, such as gold(III) chloride to a polymer, such as nylon 6, blending, and extrudingthe mixture. Additionally, T. Smith et al. report the generation ofcolloids in solution and in solid-state with metal salts, such as gold(III) chloride, in the presence of reducing agents, such as trisodiumcitrate.

Fiber-like particle scattering colorants having a colloid-like size inat least two dimensions are also preferred for certain inventionembodiments, especially where anisotropic coloration effects aredesired. One unusual method for forming very small fibers that can beused as particle scattering colorants is by the deposition of a materialwithin the confining space of a hollow nano-scale fiber. The particlescattering colorant can then either comprise the filled nano-scalediameter fiber, or the fiber of the filler that is obtained by removing(by either physical or chemical means) the sheath provided by theoriginal hollow fiber. The general approach of making such fibers by thefilling of nano-size hollow fibers is taught, for example, by V. V.Poborchii et al. in Superlattices and Microstructures, Vol. 16, No. 2,pp. 133-135 (1994). These workers showed that about 6 nm diameternano-fibers can be obtained by the injection and subsequentcrystallization of molten gallium arsenide within the 2 to 10 nmchannels that are present in fibers of chrysotile asbestos. An advantageof such small dimension particles, whether in fiber form or not, is thatthe quantum mechanical effects provide refractive indices and electronictransition energies that strongly depend upon particle size. Hence,various different coloration effects can be achieved for a particlescattering colorant by varying particle size. Also, high dichroism inthe visible can be obtained for colloidal fibers of metals andsemiconductors, and such high dichroism can result in novel visualappearances for articles that incorporate such fibers as particlescattering colorants.

Colloidal particle scattering colorants, as well as particle scatteringcolorants that have larger dimensions, that comprise an outer layer thatabsorbs in the visible are among preferred particle scattering colorantsfor use in high Δn embodiments. In such high Δn embodiments there is alarge refractive index difference between the particle scatteringcolorant and the matrix in the visible wavelength range. The reason forthis preference is that a very thin layer of a visible-light-absorbingcolorant on the outside of a colorless particle scattering colorant candramatically enhance scattering at the particle-matrix interface, whilenot substantially increasing light absorption. In order to achieve thebenefits of such particle scattering colorant configuration, it ispreferred that (1) the coating of the visible-light-absorbing coloranton the surface of the particle scattering colorant comprises on averageless than 50% of the total volume of the particles of the particlescattering colorant, (2) the average particle size of the particlescattering colorant is less that 2 microns, and (3) the refractive indexof the coating of the particle scattering colorant differs from that ofthe matrix in which the particle scattering particle is dispersed by atleast 10% at visible wavelengths. More preferably, the coating of thevisible-light-absorbing colorant on the surface of the particlescattering colorant comprises on average less than about 20% of thetotal volume of the particles of the particle scattering colorant andthe average particle size of the particle scattering colorant is lessthat 0.2 microns. Preferred applications of such surface-enhancedparticle scattering colorants are for coatings, polymer fibers, polymerfilms, and polymer molded articles. A method for the fabrication ofcolloidal particles containing a visible-light-absorbing colorant on thesurface of a colorless substrate particle is described by L. M. Gan etal. in Materials Chemistry and Physics 40, pp. 94-98 (1995). Theseauthors synthesized barium sulfate particles coated with a conductingpolyaniline using an inverse microemulsion technique. The sizes of thecomposite particles (from about 0.01 to 0.02 microns) are convenient forthe practice of the high Δn embodiments of the present invention.

Colloid particles can either be added to the matrix in the colloid-formor the colloid particles can be formed after addition to the matrix.Likewise, these processes of colloid formation and dispersion can beaccomplished for a precursor for the matrix, which is subsequentlyconverted to the matrix composition by chemical processes, such aspolymerization. For example, if the matrix is an organic polymer, suchas nylon, the metal colloids can be formed in a liquid, mixed with theground polymer, and heated above the melting point of the polymer toproduce nylon colored with particle scattering colorants. On the otherhand, either colloidal metal particles or a precursor thereof can beadded to the monomer of the polymer, the colloid particles can be formedin the monomer, and the monomer can then be polymerized. A precursor fora metal colloid can also be added to the polymer matrix and thecolloidal particles can be then formed in a subsequent step. Suchprocesses of colloidal particle formation and incorporation can befacilitated by using a melt, dissolved, gel, or solvent-swollen state ofthe polymer (or a precursor thereof) during colloid incorporation,colloid formation, or colloid formation and incorporation.Alternatively, high energy mechanical commingling involving a solidstate of the polymer (or a precursor thereof) can be used to accomplishcolloid incorporation, colloid formation, or colloid formation andincorporation.

The incorporation of colloidal size particle scattering colorants in acoating material or solution provides a preferred embodiment of thisinvention. The incorporation of colloidal size particle scatteringcolorants in the gel state of a polymer prior to the formation of saidgel state into a polymer fiber provides an additional preferredembodiment of this invention. For these processes, the particlescattering colorant should preferably have a refractive index that is atleast 10% different from that of the solid polymer matrix of the fiberat a wavelength in the visible. The average particle size of theparticle scattering colorant is preferably less than about 0.2 microns,more preferably less than about 0.08 microns, and most preferably lessthan about 0.02 microns. For particle sizes of less than about 0.02microns, the particle scattering colorants preferably significantlyabsorbs in the visible. For the case where the particle scatteringcolorant is substantially non-absorbing in the visible, the polymerfiber preferably comprises an electronic transition colorant that iscommingled with the particle scattering colorant in the gel state.Preferably this electronic transition colorant is substantially a blackcarbon form, such as carbon black, and the particle scattering colorantcomprises an inorganic composition. So as not to interfere with fiberstrength, both the particle scattering colorant and optional electronictransition colorant particle used for these fibers should have verysmall dimensions, preferably less than about 0.02 microns. Suchinvention embodiments solve a long standing problem that arises for thecoloration of high strength fibers that are spun in the gel state, suchas high molecular weight polyethylene that is spun from a mineral oilgel. This problem is that conventional organic dyes or pigments caninterfere with the formation of high quality product from the gel state.An important example of a high strength fiber product spun from the gelstate is Spectra.™.polyethylene fiber made by Honeywell (Morristown,N.J., formerly AlliedSignal). These fibers, which are gel processed athigh temperatures, are widely used for fishing lines, fishing nets,sails, ropes, and harnesses.

Ultrafine metal particles suitable for use as particle scatteringcolorants can be located on the surface of much larger particles thatare themselves particle scattering colorants. Combined-particlescattering colorants of this form are also suitable for the presentinvention. Methods for the preparation of such particle scatteringcolorants, where metal particles are deposited on much larger polymerparticles, are provided by H. Tamai et. al. in the Journal of AppliedPhysics 56, pp. 441-449 (1995). As another alternative, colloidalparticle scattering colorants can be located within larger particlesthat, depending upon their dimensions and refractive index in thevisible (relative to the matrix) can additionally provide particlescattering coloration. In any case, the larger particles are referred toas particle scattering colorants as long as the included particles areparticle scattering colorants. In a preferred case, the colloidalparticles are metal or metal alloy particles in a glass matrix. Methodsfor obtaining colloidal copper dispersed in SiO₂-comprising glass aredescribed in the Journal of Non-crystalline Solids 120, pp. 199-206(1990) and methods for obtaining silicate glasses containing colloidalparticles of various metals, including gold and silver, are described inU.S. Pat. Nos. 2,515,936; 2,515,943, and 2,651,145, which areincorporated herein by reference. These glasses containing colloidalparticle scattering colorants are transformed to particles, such as bygrinding of melt processes, and used as particle scattering colorants inembodiments of this invention. In such embodiments, these particlescattering colorants are preferably dispersed in a polymer matrix,thereby providing particle scattering coloration for articles consistingof the resulting polymer composite.

An advantage of this colloid-within-particle design of the particlescattering colorant is that the glass particles can stabilize thecolloidal particles with respect to degradation processes, such asoxidation. A second advantage is that high temperature methods can beused for forming the colloid in the glass, which could not be used forthe dispersion of the colloidal particles directly in an organic polymermatrix. A third advantage of the colloid-within-particle method is thatthe processes of colloid formation and dispersion are separated from theprocesses of dispersion of the particle scattering colorant in the finalpolymer matrix, which can provide improved process economics. A fourthadvantage is that the particle matrix can be tailored to respond toelectric/conductive, magnetic, and/or photo properties so that the colorcan be changed, substantially reduced, or both changed and substantiallyreduced when an appropriate field is applied to the coating, such as bythe color field erase device 220. As an alternative to the meltsynthesis of colloid-within-particle particle scattering colorants, suchcolorants can be synthesized by a method used by K. J. Burham et al.,which is described in Nanostructure Materials 5, pp. 155-169 (1995).These authors incorporated colloidal particles in silica by doping metalsalts in the silanes used for the sol-gel synthesis of the silicate. Bysuch means they obtained Ag, Cu, Pt, Os, Co₃ C, Fe₃ P, Ni₂ P, or Gecolloidal particles dispersed in the silica. For the purposes of thepresent invention embodiment, colloidal particles dispersed in silicacan be ground into suitable particle sizes for use as particlescattering colorants.

Instead of an inorganic glass, the particle containing the colloidparticles can be a polymer. It is known in the art to prepare films ofcolloidal dispersions of various metals in the presence of vinylpolymers with polar groups, such as poly(vinyl alcohol),polyvinylpyrrolidone, and poly(methyl vinyl ether). Particle scatteringcolorants suitable for the present invention embodiment can be obtainedby cutting or grinding (preferably at low temperatures) a polymer filmformed by solvent evaporation of the colloidal dispersion. Morepreferably, such particle scattering colorants can be formed byeliminating the solvent from an aerosol comprising colloidal particlesdispersed in a polymer-containing solvent. Particle scattering colorantsthat are either semiconductors or metallic conductors are amongpreferred compositions for use in polymer fibers. Such particlescattering colorants will generally provide significant absorption atvisible wavelengths. In such case it is preferred that the particlescattering colorant has an average diameter in the smallest dimension ofless than about 2 microns, the neat polymer matrix is substantiallynon-absorbing in the visible, and the minimum in transmitted visiblelight intensity for the particle scattering colorant is shifted by atleast by about 10 nm as a result of the finite particle size of theparticle scattering colorant. More preferably, this shift is at leastabout 20 nm for the chosen particle sizes of the particle scatteringcolorant and the chosen matrix material. For assessing the effect ofparticle size on the minimum of transmitted light intensity, a particlesize above about 20 microns provides a good approximation to theinfinite particle size limit.

For particle scattering colorant compositions that provide a singlemaximum in absorption coefficient within the visible range when particlesizes are large, another application of the standard transmitted lightintensity ratio enables the identification of preferred particlescattering colorants. This method is to identify those particlescattering colorants that have at least two minima in transmitted lightintensity ratio that occur within the visible wavelength range. Such twominima, possibly in addition to other minima, can result from either abimodal distribution of particle sizes, or differences in the minimumresulting from absorptive processes and scattering processes for amononodal distribution of particle sizes. If the particle scatteringcolorants are required for applications in which switchability incoloration states are required, it is preferable that these two minimaarise for a mononodal distribution in particle sizes. The reason forthis preference is that the switchability in the refractive indexdifference between matrix and particle scattering colorant can provideswitchable coloration if particle scattering effects are dominant. Thus,in one embodiment of this invention, this switchable coloration due toswitchability in the refractive index is combined with changes or lossin coloration due to agglomeration of particles. Mononodal and bimodalparticle distributions, referred to above, designate weight-fractionparticle distributions that have one or two peaks, respectively.

For applications in which reversible color changes in response totemperature changes are desired, particular ceramics that undergoreversible electronic phase changes are preferred particle scatteringcolorants for the present invention. Such compositions that undergoreversible transitions to highly conducting states upon increasingtemperature are VO₂, V₂O₃, NiS, NbO₂, FeSi₂, Fe₃ O₄, NbO₂, Ti₂ O₃, Ti₄O₇, Ti₅ O₉, and V_(1−x) M_(x) O₂, where M is a dopant that decreases thetransition temperature from that of VO₂ (such as W, Mo, Ta, or Nb) andwhere x is much smaller than unity. VO₂ is an especially preferredcolor-changing particle additive, since it undergoes dramatic changes inboth the real and imaginary components of refractive index at aparticularly convenient temperature (about 68° C.). The synthesis andelectronic properties of these inorganic phases are described by Specket al. in Thin Solid Films 165, 317-322 (1988) and by Jorgenson and Leein Solar Energy Materials 14, 205-214 (1986).

Because of stability and broad-band ability to absorb light, variousforms of aromatic carbon are preferred electronic transition colorantsfor use in enhancing the coloration effects of particle scatteringcolorants. Such preferred compositions include various carbon blacks,such as channel blacks, furnace blacks, bone black, and lamp black.Depending upon the coloration effects desired from the combined effectsof the particle scattering colorant and the electronic colorant, variousother inorganic and organic colorants that are conventionally used bythe pigment and dye industry are also useful. Some examples of suchinorganic pigments are iron oxides, chromium oxides, lead chromates,ferric ammonium ferrocyanide, chrome green, ultramarine blue, andcadmium pigments. Some examples of suitable organic pigments are azopigments, phthalocyanine blue and green pigments, quinacridone pigments,dioxazine pigments, isoindolinone pigments, and vat pigments.

The use of either electronic transition colorants that are dichroic or adichroic matrix composition can be used to provide novel appearances.Such novel appearances can result, for example, since the scattering ofparticle scattering colorants can display a degree of polarization.Preferential orientation of the dichroic axis is preferred, preferablyeither parallel or perpendicular to the fiber axis for a fiber or in thefilm plane for a film, and can be conveniently achieved byconventionally employed methods used to make polarizers, such asmechanical drawing. The dichroic behavior can be usefully developedeither in the same matrix component in which the particle scatteringcolorant is dispersed or in a different matrix component. One preferredmethod for providing dichroic polymer matrix materials for the large Δnembodiments is by incorporating a dye molecule in the polymer, followedby uniaxially stretching the matrix containing the dye molecule. Such adye molecule serves as a dichroic electronic absorption colorant. Theeffect of the mechanical stretching process is to preferentiallyorientate the optical transition axis of the dye molecule with respectto the stretch axis of the polymer. The creation of polarizing films bythe mechanical stretching of a polymer host matrix is described by Y.Direx et al. in Macromolecules 28, pp. 486-491 (1995). In the exampleprovided by these authors, the dye was sudan red and the host matrix waspolyethylene. However, various other combinations of dye molecules andpolymer matrices are suitable for achieving the polarizing effect thatcan be usefully employed in the particle scattering colorant compositesof the present invention embodiments.

Various chemical compositions that are capable of providingswitchability in refractive index or adsorption coefficients are usefulfor either host matrices, particle scattering colorants, or electronictransition colorants that enhance the effects of scattering particlecolorants. In order to achieve novel coloration effects that areanisotropic, all of these switchable chemical compositions that areanisotropic can optionally be incorporated in a preferentiallyorientated manner in fabricated articles. By providing refractive indexand electronic transition changes that occur as a function of thermalexposure, light exposure, or humidity changes, such materials (eitherwith or without preferential orientation) provide a switchablecoloration state. A host of such color-changing chemicals suitable forthe present invention are well known, such as the anils, fulgides,spiropyrans, and other photochromic organics described in the book by A.V. El'tsov entitled “Organic Photochromes” (Consultants Bureau, NewYork, 1990). Such color changing chemicals can be employed as electronictransition colorants that modify the visual effect of particlescattering colorants in polymer composites. Also, color changes inresponse to temperature, light exposure, or humidity can alternativelybe produced by using the many well-known materials that providerefractive index changes in response to these influences, and nosignificant change in absorption coefficients at visible lightwavelengths. Such materials can be used as either the matrix material orthe particle scattering colorants for the color changing composites.

A host of photopolymerizable monomers, photo-dopable polymers,photo-degradable polymers, and photo cross-linkable polymers are alsoavailable for providing the switchable refractive indices and switchableelectronic absorption characteristics that enable the construction ofarticles having switchable particle scattering coloration. Materialssuitable for this use are described, for example, in Chapter 1 (pages1-32) written by J. E. Lai in the book entitled “Polymers for ElectronicApplications”, which is also edited by the same author (CRC Press, BocoRaton, Fla., 1989). Improved materials that are now being introduced aredescribed by G. M. Wallraff et al. in CHEMTECH, pp. 22-30, April 1993.More exotic compositions suitable for the present application aredescribed by M. S. A. Abdou, G. A. Diaz-Guijada, M. I. Arroyo, and S.Holdcroft in Chem. Mater. 3, pp. 1003-1006 (1991).

Coatings or polymer colored articles of the present technology can alsocontain fillers, processing aids, antistats, antioxidants, antiozonants,stabilizers, lubricants, mold release agents, antifoggers, plasticizers,and other additives standard in the art. Unless such additivesadditionally serve desired purposes as particle scattering colorants orelectronic transition colorants, such additives should preferably eitherdissolve uniformly in the coating or polymer that contains the particlescattering colorant or such additives should have a degree oftransparency and a refractive index similar to the matrix polymer.Dispersing agents such as surfactants are especially useful in thepresent invention for dispersing the particle scattering colorantparticles. Many suitable dispersing agents and other polymer additivesare well known in the art and are described in volumes such as“Additives for Plastics”, edition 1, editors J. Thuen and N. Mehlberg(D.A.T.A., Inc., 1987). Coupling agents that improve the couplingbetween particle scattering particles and host matrix are especiallyimportant additives for vanishing Δn embodiments, since they caneliminate fissure formation or poor wetting at particle-matrixinterfaces. For cases where either a glass or a ceramic is the particlescattering colorant, and the host matrix is an organic polymer,preferred coupling agents are various silanes that are commerciallyavailable and designed to improve bonding in composites that involveboth inorganic and organic phases. Examples of suitable coupling agentsfor particle scattering colorant composites of this type are 7169-45Band XI-6124 from Dow Corning Company.

The colored articles of the present invention can optionally containmaterials that are either fluorescent or phosphorescent. An example ofsuch known materials are of the form Zn_(1−x) Cd_(x) S, where x is nogreater than unity, that contains Cu, Ag, or Mn impurities.

In various teachings of this invention we refer to photopolymerizablemonomers and oligomers. Examples of such compositions that are suitablefor the practice of invention embodiments are monomers containing two ofmore conjugated diacetylene groups (that are polymerizable in the solidstate), vinyl ether terminated esters, vinyl ether terminated urethanes,vinyl ether terminated ethers, vinyl ether terminated functionalizedsiloxanes, various diolefins, various epoxies, various acrylates, andhybrid systems involving mixtures of the above. Various photoinitiatorsare also useful for such systems, such as triarylsulfonium salts.

Various methods can be employed for the compounding and fabrication ofthe composites of the present invention. For example, particlescattering colorants can be compounded with polymeric matrix materialsvia (1) melt-phase dispersion, (2) solution-phase dispersion, (3)dispersion in a colloidal polymer suspension, or (4) dispersion ineither a prepolymer or monomer for the polymer. Films of the compositecan be either formed by solvent evaporation or by adding a non-solventto a solution containing dispersed ceramic powder and dissolved polymerfollowed by sample filtration, drying, and hot pressing. In method (4),the ceramic particles can be dispersed in a monomer or prepolymer thatis later thermally polymerized or polymerized using actinic radiation,such as ultraviolet, electron-beam, or .gamma.-ray radiation. Particlescattering colorants can also be combined with the matrix byxerographic, power coating, plasma deposition, and like methods that arewell known in the art. For example, particle scattering colorants can beadded to fabrics or carpet by using xerography techniques described in“Printing Textile Fabrics with Xerography” (W. W. Carr, F. L. Cook, W.R. Lanigan, M. E. Sikorski, and W. C. Tinche, Textile Chemist andColorist, Vol. 23, no. 5, 1991). The coating of textile, carpet fiber,and wallpaper articles with particle scattering colorants in a fusiblepolymer matrix, so as to obtain coloration, is an especially importantembodiment because of the commercial importance of speedy delivery ofarticles that accommodate frequent style and color changes andindividual customer preferences. Such deposition can optionally bepreceded by a separate deposition of an electronic transition colorantin order to enhance the effect of the particle scattering colorant.

In order to obtain uniform mixing of the ceramic in the host polymer,ultrasonic mixers can be used in the case of low viscosity compositeprecursor states and static mixers and more conventional mixers can beused for melt blending processes. Static mixers, which are particularlyuseful for melt blending processes, are available commercially fromKenics Corporation of Danvers, Mass., and are described by Chen andMacDonald in Chemical Engineering, Mar. 19, 1973, pp. 105-110.Melt-phase compounding and melt-phase fabrication are preferred for thecompositions of the present invention. Examples of useful melt-phasefabrication methods are hot rolling, extrusion, flat pressing, andinjection molding. For the fabrication of the more complicated shapes,injection molding and extrusion are especially preferred.

In some cases it is desirable to achieve a degree of controlledaggregation of the particle scattering colorants in order to achieveanisotropy in coloration effects. Such aggregation to produce anisotropyin coloration is preferably in either one dimension or two dimensions,wherein the direction of such aggregation for different particleaggregates are correlated. Such correlation in aggregation is mostconveniently achieved by plastic mechanical deformation of a matrix thatis heavily loaded with the particle scattering colorant. For example,such mechanical deformation can be in the fiber direction for a fiber orin either one or both of two orthogonal directions in the film plane fora film. As an alternative to using particle aggregation to achieveanisotropy in coloration, anisotropy in particle shape can be used toachieve the similar effects. For example, mechanical deformation offilms and fibers during processing will generally cause plate-likeparticles to preferentially orient with the plate plane orthogonal tothe film plane and fiber-like particles to preferentially orient withthe particle fiber axis parallel to the fiber axis of the composite.

A special type of particle scattering colorant orientation effect isespecially useful for vanishing Δn embodiments. In such embodiments itis usually preferred that the particle scattering colorants and matrixmaterials are isotropic in optical properties. However, in order toobtain novel angle-dependent coloration effects, one can preferentiallyorient plate-like particles of an anisotropic particle scatteringcolorant in polymer films so that an optic axis of the particles isnormal to the film plane. Such particles and polymer matrix are chosenso that the ordinary refractive index (n_(o)) of the particles equalsthat of the matrix at a wavelength in the visible. Hence, a film articlewill appear highly colored when light perpendicular to the film plane istransmitted through the film. However, light that is similarly viewedthat is inclined to the film plane will be scattered at all wavelengthsso the article will appear either uncolored or less intensely colored.In such embodiments the particle scattering colorant is chosen to be onethat has the optic axis perpendicular to the particle plate plane, whichis the case for many materials having either hexagonal, trigonal, ortetragonal symmetry. Preferential orientation of the plane of theplate-like particles parallel to the film plane can be obtained byvarious conventional processes, such as film rolling processes, filmformation by solution deposition processes, and biaxial stretchingprocesses. Note that such plate-like particle scattering colorants arequite different from the plate interference colorants of the prior art.For these prior art colorants, no match of refractive indices of matrixand particle is required, and, in fact, large refractive indexdifferences between the particles and the matrix throughout the visiblecan increase the coloration effect.

Fibers of the present invention embodiments can either be formed byconventional spinning techniques or by melt fabrication of a filmfollowed by cutting the film into either continuous fibers or staple. Anelectronic transition colorant can be optionally included in thecomposite film composition. Alternately, a polymer film containing theparticle scattering colorant can be adhesively joined either to one sideor to both sides of a polymer film that contains an electronictransition colorant. The adhesive tie layer between these polymer filmlayers can be any of those typically used for film lamination. However,it is preferable to employ the same matrix polymer for the joined filmsand to select the tie layer to have about the same refractive index asthis matrix polymer. Alternately, the central film layer containingelectronic transition colorant and the outer film layers containing theparticle scattering colorant can be coextruded in a single step usingwell-known technologies of polymer film coextrusion. If the desired endproduct is a polymer fiber, these multilayer film assemblies can besubsequently cut into fiber form. Microslitter and winder equipment isavailable from Ito Seisakusho Co., Ltd (Japan) that is suitable forconverting such film materials to continuous fibers. Particularlyinteresting visual effects can be obtained if these fibers are cut froma bilayer film that consists of a polymer film layer containing theparticle scattering colorant on one side and a polymer film layercontaining an electronic transition colorant on the opposite side. Suchfibers that provide a different visual appearance for different viewingangles can be twisted in various applications, such as carpets andtextiles, to generate a spatially colored material due to the appearancein one viewing angle of alternating segments with different coloration.One coloration effect is provided if the fiber side that is in closestview is the particle scattering colorant film layer and anothercoloration effect is provided if the side that is in closest view is theelectronic transition colorant film layer. Such special colorationeffects of cut film fibers are most visually noticeable if the cut filmfiber strips have a width-to-thickness ratio of at least 5.Additionally, dimensional compatibility of such fiber for comminglingwith conventional polymer fibers in textile and carpet applications isincreased if the cut film fibers have a denier that is less than 200. Asan alternative to the slit-film process, either bilayer or multilayerfibers having these characteristics can be directly melt spun using aspinneret that is designed using available technology of spinnerets.This paragraph has emphasized the formation of fibers by the cutting ofpolymer films of this invention that provide particle scatteringcoloration effect. However, it should be emphasized that the films thathave been described also provide important commercial opportunities asfilm products for application in diverse areas, from product packagingon one extreme to wallpaper on another.

Sheath-core fibers which are suitable for the invention are fiberscomprising a sheath of a first composition and a core of a secondcomposition. Either the sheath or the core can be organic, inorganic, ormixed inorganic and organic, independent of the composition of the othercomponent. Preferably both the sheath and core of such fibers containorganic polymer compositions. Also, the particle scattering colorant ispreferably located in the sheath and an electronic transition colorantis preferably located in the core. By choice of either sheath or corecross-sectional geometry that does not have circular cylindricalsymmetry, it is possible to provide fibers that provide differentcolorations when viewed in different lateral directions. For example,the external sheath geometry can be a circular cylinder and the core canbe an ellipse having a high aspect ratio. When viewed orthogonal to thefiber direction along the long axial direction of the ellipse, theeffect of the electronic transition colorant can dominate coloration. Onthe other hand, a corresponding view along the short axis of the ellipsecan provide a visual effect that is less influenced by the electronictransition colorant. More generally, in order to achieve such angledependent visual effects the maximum ratio of orthogonal axialdimensions in cross-section for the outer surface of the sheath ispreferably less than one-half of the corresponding ratio for the core.Alternatively, the sheath and core should preferably both have a maximumratio of orthogonal axial dimensions in cross-section that exceeds twoand the long-axis directions in cross-section of sheath and core shouldpreferably be unaligned. Such fibers that provide a different visualappearance for different viewing angles can be twisted in variousapplications, such as carpets and textiles, to generate a spatiallycolored material whose appearance in one viewing angle is determined byalternating segments with different coloration.

The ability to change the coloration of sheath-core fibers by varyingthe relative cross-sections of sheath and core provides for theconvenient fabrication of yarns that display interesting visual effectsbecause of variations in the coloration of different fibers in the yarn.Such variation can be accomplished, for example, by varying the relativeor absolute sizes of the sheath and cores, their relative shapes, andthe relative orientation of the sheath and core cross-sections. For anyof these cases, the said variation can be provided either along thelength of individual fibers or for different fibers in a yarn.Preferably in these embodiments, the particle scattering colorant is inthe fiber sheath and an electronic transition colorant is in the fibercore. Also, a yarn consisting of such fibers is preferably assembleddirectly after spinning from a multi-hole spinneret. Variation in theindividual spinneret hole constructions, or variation in the feedpressures for the sheaths and cores for different fiber spinning holes,can permit the desired fiber-to-fiber variations in either sheathcross-section, the core cross-section, or both. Alternatively, variationin the coloration of individual fibers along their length can beachieved by convenient means. These means can, for example, be byvarying as a function of spinning time either (1) either the sheathpolymer feed pressure or the core polymer feed pressure or (2) therelative temperatures of the sheath and the core polymers at thespinneret. Of these methods, variation in coloration along the lengthsof individual fibers is preferred, and such variations are preferablyachieved by changing the relative feed pressures of the sheath and corefiber components. Such pressure variations are preferably accomplishedsimultaneously for the spinneret holes that are used to producedifferent fibers and such spinneret holes for different fibers arepreferably substantially identical. Yarns are preferably formed from thefibers at close to the point of spinning, so that correlation in thelocation of like colors for different fibers is not lost. As a result ofsuch preferred embodiment, the color variations of individual fibers arespatially correlated between fibers, so these color variations are mostapparent in the yarn.

The fact that fiber coloration depends upon both the sheath/core ratioand mechanical draw processes when the particle scattering colorant isin the sheath and the electronic transition colorant is in the coreprovides important sensor applications. These sensor applicationsutilize the coloration changes resulting from fiber wear and other fiberdamage processes, such as the crushing of fibers which can providecoloration by deforming the cross-sections of sheath and core, abrasionor fiber dissolution which can change the cross-section of the fibersheath, and fiber stretching (which can change the cross-sections ofsheath and core, provide particle scattering colorant aggregation, andincrease both polymer chain orientation and fiber crystallinity). In anycase, the basis for these color changes is generally a changing relativecontribution from particle scattering colorant and electronic transitioncolorant to article coloration. Such sensors can provide valuableindication of damage in articles such as ropes, slings, and tire cordwhere the possibility of catastrophic failure and uncertainties in whensuch failure might occur lead to frequent article replacement. Thesesheath/core fibers can be used either as a color-indicating minority ormajority fiber in such articles.

Special methods of this invention can be used to obtain particle-inducedcoloration for fibers that are spun in hollow form. The particles thatprovide coloration via scattering can be dispersed in a suitable liquid,which subsequently fills the hollow fibers. Optional electronictransition colorants can be included in this liquid in order to enhancethe coloration effect. This approach is enabled by using either aprecursor fiber that is staple (i.e., short open-ended cut lengths) orto use hollow fibers that contain occasional micro holes, where thehollow fiber core breaks to the surface. The existence of these microholes enables rapid filling of the fibers. Modest pressures ofpreferably less than 2000 psi can be used to facilitate rapid filling ofthe fibers. A low viscosity carrier fluid is preferably chosen as onethat can be either photopolymerized or thermally polymerized after thefilling process. As an alternative to this approach, the particlescattering colorant can be included in molten polymer from which thehollow fibers are melt spun. Then the polymerizable fluid that is drawninto the hollow fiber after spinning can include an electronictransition colorant for enhancing the coloration effect of the particlescattering colorant. Various modifications of these methods can beemployed. For example, melt spun fibers can contain various combinationsof particle scattering and electronic transition colorants, as can thefluid that is drawn into the hollow fibers. As another variation ofthese methods, hollow fibers spun from a melt that contain a particlescattering colorant can be coated on the interior walls with a materialthat absorbs part of the light that is not scattered by the particlescattering colorant. For example, such coating can be accomplished bydrawing an oxidant-containing monomer solution for a conducting polymer,solution polymerizing the conducting polymer onto the interior walls ofthe hollow fibers, and then withdrawing the solution used forpolymerization from the hollow fibers. The inner walls of hollow fibersare preferably colored with an electronic transition colorant using asolution dye process that requires thermal setting. For example, a dyesolution can be imbibed into the hollow fibers by applying suitablepressure, any dye solution on the exterior surface of the fibers can bewashed away, the dye coloration can be set by thermal treatment, and thedye solution contained within the fibers can be removed (such as byevaporation of an aqueous solution). As an alternative to thermalsetting, the setting of the dye on the inner surface of the hollowfibers can be by either photochemical or heating effects of radiation,such as electron beam, ultraviolet, or infrared radiation. Such thermalor photoassisted setting of the dye can be accomplished in a patternedmanner, thereby providing fibers that display the type of spatialcoloration effects that are sought after for carpet and textileapplications.

The same methods above described for obtaining internal wall dyeing ofhollow fibers can be used for the achievement of novel optical effectsvia deposition of particle scattering colorants on the inside of hollowfibers. These particle colorants are preferably deposited by imbibing acolloidal solution containing the particle scattering colorant into thehollow fibers and then evaporating the fluid that is the carrier for thecolloidal particles. The liquid in which the colloidal particles aredispersed can optionally contain a material that forms a solid matrixfor the colloidal particles after fluid components are eliminated. Suchcolloidal particle scattering colorants, whether deposited on the innerwalls as a neat layer or as a dispersion in a matrix, can then beoptionally coated with an electronic transition colorant by methodsdescribed above for coating the inner walls of hollow fibers that arenot coated with particle scattering colorants. Note that the abovedescribed deposition of colloidal particles on the inside of hollowfibers can result in aggregation of these particles to the extent thatthey transform from particle scattering colorants to electronictransition colorants. In this invention this aggregation can be enhancedby selecting particles which respond to electric/conductive, magnetic,and/or photo properties so that the color can be changed, substantiallyreduced, or both changed and substantially reduced when an appropriatefield is applied, such as by the color field erase device 220.

In the following embodiment of this invention, particle scatteringcolorants are used in hollow fibers to produce photochromism. Suchphotochromism can be achieved using particle scattering colorants thatare photoferroelectrics. Preferred photoferroelectrics for thisapplication are, for example, BaTiO₃, SbNbO₄, KNbO₃, LiNbO₃, and suchcompositions with optional dopants such as iron. These and relatedcompositions are described in Chapter 6 (pp. 85-114) of“Photoferroelectrics” by V. M. Fridkin (Springer-Verlag, Berlin, 1979).Photovoltages of the order 10³ to 10⁵ volts can be generated forphotoferroelectrics, although it should be recognized that thesephotovoltages decrease as the particle size in the polarizationdirection decreases. The corresponding photo-generated electric fieldscan be used to reversibly produce aggregation (i.e., particle chaining)of photoferroelectric particles that are dispersed in a low conductivityliquid within the cavity of a hollow fiber. If these photoferroelectricparticles have suitably small dimensions, aggregation and deaggregationprocesses will provide a photo-induced change in the visual appearanceand coloration of the fiber. The electrical conductivity of the fluidcan determine the rate of return of the coloration to the initial stateafter light exposure ceases, since this conductivity can lead to thecompensation of the photo-induced charge separation that provides thephoto-induced field. Methods described above can be used for the fillingof the hollow fibers with the photoferroelectric-containing liquid, andsuch liquid can be sealed in the fibers by a variety of processes, suchas by periodic closure of the hollow tubes using mechanical deformation.Articles consisting of these photochromic fibers can be used for variousapplications, such as clothing that automatically changes color uponlight exposure.

In another invention embodiment, the particle scattering colorant is aphotoferroelectric that is dispersed in a solid matrix that has the samerefractive index as the photoferroelectric at some wavelength in thevisible (either when the photoferroelectric is not exposed to light orafter it has been exposed to light, or both). This embodiment uses thelarge refractive index changes that occur upon the exposure of aphotoferroelectric to light, which shifts the wavelength at whichrefractive index matching occurs (or either causes or eliminates suchrefractive index matching), thereby causing a coloration change inresponse to light.

In previously discussed embodiments of this invention (for sheath-corefibers, trilayer and bilayers films and derived cut-film fibers, andhollow polymer fibers), the use of particle scattering colorants in alayer that is exterior to the layer containing an electronic transitioncolorant has been described. One described benefit is the novelcoloration effects achieved. Another benefit of such configurations isparticularly noteworthy. Specifically, particle scattering colorantsthat provide blue coloration also generally provide significantscattering in the ultraviolet region of the spectra that can cause thefading of many electronic transition colorants. Hence, this ultravioletscattering can protect the underlying electronic transition colorantsfrom fading due to ultraviolet light exposure.

Preferred embodiments result from the advantages of using a particlescattering colorant to provide ultraviolet light protection forultraviolet-light sensitive fiber and film products. For articles inwhich the particle scattering colorant is dispersed in a first matrixmaterial that is substantially exterior to a second matrix componentcomprising an electronic transition colorant (such as for abovedescribed hollow fibers, sheath-core fibers, and trilayer films andderived cut-film fibers) it is preferred that (1) the first matrixcomponent and materials contained therein absorb less than about 90% ofthe total visible light that can be incident on the article from atleast one possible viewing angle, (2) the absorption coefficient of thefirst matrix component and materials contained therein is less thanabout 50% of that of the second matrix component and materials containedtherein at a wavelength in the visible, (3) and the particle scatteringcolorant is substantially non-absorbing in the visible. In addition, itis preferable that the first matrix component and materials containedtherein either absorb or scatter more than about 50% of uniformradiation at the ultraviolet wavelength at which the second matrixcomponent comprising the electronic dopant undergoes the maximum rate ofcolor fading. The term uniform radiation means radiation that has thesame intensity for all spherical angles about the sample. Uniformradiation conditions exist if there is the same radiation intensity forall possible viewing angles of the article. The average particle sizethat is most effective for decreasing the transmission of light througha matrix at a wavelength λ_(o) is generally greater than about λ_(o)/10and less than about λ_(o)/2. Hence, for maximum protection of anelectronic transition colorant that most rapidly fades at λ_(o), theaverage particle for the particle scattering colorant should preferablybe from about λ_(o)/2 to about λ_(o)/10. Additionally, for this purposethe particle scattering colorant should preferably be approximatelyspherical (having an average ratio of maximum dimension to minimumdimension for individual particles of less than four) and there shouldbe little dispersion in the sizes of different particles. Mostpreferably the average particle size for the particle scatteringcolorants used for ultraviolet light protection of electronic transitionpigments should be from about 0.03 to about 0.1 microns. Particlescattering colorants that are especially preferred for conferringultraviolet light protection for electronic transition colorants aretitanium dioxide and zinc oxide.

Materials suitable for the present art include inorganic or organicmaterials that have any combination of organic, inorganic, or mixedorganic and inorganic coatings. The only fundamental limitation on sucha coating material is that it provides a degree of transparency in thevisible spectral region if the entire surface of the article is coveredwith such a coating material. Preferred coating materials forapplication to film, fiber, or molded part surfaces are well-knownmaterials that are called antireflection coating materials, since theyminimize the reflectivity at exterior surfaces. Such antireflectioncoatings can enhance the visual effect of particle scatting colorants bydecreasing the amount of polychromatically reflected light.Antireflection coatings can be provided by applying a coating to thesurface of an article so that the refractive index of the coating isclose to the square root of the refractive index of the surface of thearticle and the thickness of the coating is close to λ/4, where λ is theapproximate wavelength of light that is most problematic. For example,antireflection coatings can be obtained by well known means for polymerssuch as polycarbonate, polystyrene, and poly(methyl methacrylate) byfluorination of the surface, plasma deposition of fluorocarbon polymerson the surface, coating of the surface with a fluoropolymer fromsolution, or in situ polymerization of a fluoromonomer that has beenimpregnated on the surface. Even when the refractive index of theantireflection polymer layer does not closely equal the square root ofthe refractive index of the surface of the article, light is incident atan oblique angle to the surface, and the wavelength of the lightsubstantially deviates from λ, antireflection properties suitable forthe present application can be obtained using such single layers.Furthermore, the known technologies of broadband, multilayerantireflection coatings can be used to provide antireflection coatingshaving improved performance. Hence, antireflection coatings can beprovided for essentially any substrate, such as a polymer film, thatdecrease the polychromatic surface reflection that can interfere withthe visual effect of particle scattering colorants.

The ability to arrange the light scattering particles in a patternedmanner is important for achieving the spatial coloration that isdesirable for many articles, such as polymer fibers. A number ofprocesses can be used to achieve such spatial coloration. One method isto use the effect of magnetic fields on ordering magnetic colloidalfluids, such fluids being transformable into solid materials by thermalor photochemical setting. Such thermal setting is preferably either bydecreasing temperature to below a glass transition or meltingtemperature or by thermal polymerization. Such photochemical setting ispreferably by photo-polymerization to a glassy state. Another usefulsetting process is solvent evaporation from the colloidal suspension.Such setting should be substantially accomplished while the magneticmaterial is in a magnetic-field-ordered state, so that novel opticalproperties are conferred on the article by scattering and absorptiveeffects of the ordered magnetic material. Examples of magnetic colloidalsuspensions that can be used to provide novel coloration effects areeither water-based or organic-based suspensions of nanoscale magneticoxides. Such suspensions, called ferrofluids, are obtainablecommercially from Ferrofluidics Corporation, Nashua N.H. and aredescribed by K. Raj and R. Moskowitz in the Journal of Magnetism andMagnetic Materials, Vol. 85, pp. 233-245 (1990). One example of howmagnetic particles can be deposited in a spatially variant way isindicated by returning to the above examples of hollow fibers. Suchhollow fibers can be filled with a dispersion of the magnetic particlesin a polymerizable fluid. The magnetic particles can be spatiallydistributed in a desired pattern along the length of the hollow fibersusing a magnetic field. Finally, the fluid can be polymerized orcross-linked thermally or by exposure to actinic radiation in order toset the structure. Polyurethane thermosets provide one preferred type ofthermally set fluid for this application.

Spatially variant coloration of fibers and films can be accomplishedquite simply by mechanical drawing processes that vary along the lengthof the fiber or film. Variation in the degree of draw can providevariation in the refractive index of the polymer matrix and the degreeof stretch-induced crystallinity. These variations provide spatiallydependent variation in the coloration resulting from particle scatteringcolorants. For such spatially dependent variation of coloration to bevisually perceived, predominant color changes should occur lessfrequently than every 200 microns, unless the separation between regionshaving different optical properties is sufficiently short to providediffraction grating or holographic-like effects.

Especially interesting and attractive visual effects can be achieved bythe deposition of particle scattering colorants as a pattern that isspatially variant on the scale of the wavelength of light. The result ofsuch patterning is the creation of a holographic-like effect. Thepreferred particle scattering colorants of the present inventionembodiment have refractive indices for all wavelengths in the visiblespectra-which do not equal those of the host matrix at the samewavelength, which is in contrast with the case of Christiansen filters.In fact, it is preferable that the particle scattering colorants thatare patterned to provide the holographic effect differ from that of thematrix by at least about 10% throughout the visible region. Mostpreferably, this difference in refractive index of particle scatteringcolorant and host matrix is at least about 20% throughout the visibleregion of the spectra.

The effect of the particle scattering colorants on the coloration ofpolymer articles and the polymers contained therein can be dramaticallydecreased or even eliminated, which is an important advantage of thepresent technology—since it enables the recycling of originally coloredpolymers to provide polymer resin that has little or no coloration.Special embodiments of the present invention enable such recycling. Inthe first embodiment both particle scattering colorants and electronictransition colorants are employed in different matrix polymers, so thatthe coloration effect of the particle scattering colorant is substantialonly in the presence of the electronic transition colorant (whichabsorbs non-scattered light so that this light does not interfere withthe visual effect of light that is chromatically scattered by theparticle scattering colorant). In this embodiment, the scatteringcolorant has no significant absorption in the visible (or at least nosignificant absorption peak in the visible) and the matrix polymer forthe particle scattering colorant and electronic transition colorant aresufficiently different that separation by physical or chemical means isviable. For example, this separation can be accomplished by eitherdissolving only one of the matrix polymers or causing the matrix polymerfor the electronic transition colorant to depolymerize.

The second embodiment employs colored articles that preferably containonly a particle scattering colorant. In this type of recycling methodthe coloration of the polymer is either decreased or eliminated byeither (1) a thermal heating or irradiation process that eitherdecreases the refractive index difference between particle scatteringcolorant and the host matrix to a value that is small, but eithernon-zero anywhere in the visible range or substantially zero throughoutall the visible range; (2) a thermal heating or irradiation process thateither eliminates a match of refractive index between matrix andparticle scattering colorant at a wavelength in the visible or causessuch match to occur over a broad spectral range; or (3) either adissolution, evaporation, or chemical process that removes the particlescattering colorant from the host matrix. For example, the particlescattering colorant can be an organic composition that evidences a highrefractive index with respect to the matrix because of the presence ofdouble bonds. Chemical processes (such as ultraviolet-induced,four-centered coupling of double bonds to form cyclobutane rings) candramatically decrease the refractive index difference of the particlesand the matrix, thereby effectively eliminating the coloration. Asanother example, the particle scattering colorant can be chosen as onethat is sublimable at temperatures at which the matrix polymer isthermally stable, one that is soluble in solvents that are non-solventsfor the matrix polymer, or one that dissolves in the matrix polymer. Inall of these cases, the coloration of the polymer is either decreased oreliminated by either destroying the particles, decreasing the refractiveindex difference between the particles and the matrix, eliminating aperfect match of refractive indices of the particles and the matrix atonly one wavelength, or separating the particles from the matrixpolymer. In fact, methods above described for obtaining switchablecoloration of polymers (via refractive index changes), which are usefulfor obtaining spatial coloration effects in polymer articles, are alsouseful for either decreasing or eliminating coloration during recyclingprocesses. A third embodiment of this invention for providing recyclablecolored polymers uses mechanical processes, such as polymer grinding,that cause either aggregation or stress-induced chemical reaction of theparticle scattering colorant, thereby eliminating the effectiveness ofthe particle scattering colorants for providing coloration.

The particle scattering colorant embodiments of the present inventionthat are described above do not necessarily require the arrangement ofthe individual particles as an array having translational periodicity.Such arrangement is sometimes desirable, since novel visual appearancescan result, especially intense iridescent coloration. The problem isthat it has been so far impossible to achieve such periodic arrangementsin either the desired two or three dimensions on a time scale that isconsistent with polymer processing requirements, which are dictated byeconomics. The presently described invention embodiment provides aneconomically attractive method to achieve these novel visual effects forpolymers. The particle scattering colorants of this embodiment consistof primary particles that are arranged in a translationally periodicfashion in m dimensions, where m is either 2 or 3. At least onetranslational periodicity of the particle scattering colorants ispreferably similar to the wavelength of light in the visible spectrum.More specifically, this preferred translational periodicity is fromabout 50 to about 2000 nm. More preferably this translationalperiodicity is from about 100 to about 1000 nm. In order to obtain suchtranslational periodicity, it is desirable for the particle scatteringcolorant to consist of primary particles that have substantially uniformsizes in at least m dimensions. The particle scattering colorant canoptionally comprise other primary particles, with the constraint thatthese other primary particles are either small compared with the abovesaid primary particles or such other primary particles also haverelatively uniform sizes in at least the said m dimensions. The averagesize of the primary particles in their smallest dimension is preferablyless than about 500 nm.

The first step in the process is the preparation of translationallyordered aggregates of the primary particles. Since this first step doesnot necessarily occur on the manufacturing lines for polymer articles,such as fibers, films, or molded parts, the productivity of suchmanufacturing lines need not be reduced by the time required for theformation of particle scattering colorants consisting of translationallyperiodic primary particles. The second step in the process is tocommingle the particle scattering colorant with either the polymer hostmatrix or a precursor thereof Then, as a third step or steps, any neededpolymerization or crosslinking reactions can be accomplished andarticles can be fashioned from the matrix polymer containing theparticle scattering colorant particles. In order to optimize desiredvisual effects, it is critically important that such second and thirdstep processes do not completely disrupt the translationally periodicarrangement of primary particles within the particle scatteringcolorants. This can be insured in a number of ways. First, the averagesize of the particle scattering colorant particles in the smallestdimension should preferably be less than about one-third of the smallestdimension of the polymer article. Otherwise mechanical stresses duringarticle manufacture can disrupt the periodicity of the primary particlesin the particle scattering colorant. The particle scattering colorantdimension referred to here is that for the particle scattering colorantin the shaped polymer matrix of the polymer article. However, it is alsopreferable that the particle sizes of the particle scattering colorantin the fashioned polymer matrix of the polymer article are thoseinitially formed during the aggregation of the arrays of primaryparticles. The point is again that mechanical steps, such as mechanicalgrinding, should be avoided to the extent possible if these stepspotentially disrupt the translation periodicity within the particlescattering colorant, such as by the production of cracks or grainboundaries within the particle scattering colorant.

Various methods can be used for the first step of forming the particlescattering colorant particles containing translationally periodicprimary particles. One useful method is described by A. P. Philipse inJournal of Materials Science Letters 8, pp. 1371-1373 (1989). Thisarticle describes the preparation of particles having an opal-likeappearance (having intense red and green scattering colors) by theaggregation of silicon spheres having a substantially uniform dimensionof about 135 nm. This article also teaches that the mechanicalrobustness of such particle scattering colorant having a threedimensionally periodic arrangement of silica spheres can be increased byhigh temperature (a few hours at 600° C.) treatment of the silica sphereassembly. Such treatment decreased the optical effectiveness of theparticle scattering colorant, since the particles became opaque.However, Philipse taught that the particle aggregates recover theiroriginal iridescent appearance when immersed in silicon oil for a fewdays. Such treatment (preferably accelerated using either appliedpressure, increased temperature, or a reduced viscosity fluid) can alsobe used to produce the particle scattering colorant used for the presentinvention. However, it is more preferable if the mechanical robustnessis achieved by either (1) forming the translationally periodic assemblyof spherical primary particles from a fluid that can be latterpolymerized, (2) either imbibing or evaporating a fluid to inside theas-formed translationally periodic particle assembly and thenpolymerizing this fluid, or (3) annealing the translationally periodicparticle assembly (as done by Philipse), either imbibing or evaporatinga fluid in inside this particle assembly, and then polymerizing thisfluid. Alternatively, materials can be dispersed inside the periodicarray of primary particles by gas phase physical or chemical deposition,such as polymerization from a gas phase. Such methods and relatedmethods that will be obvious to those skilled in the art can be employedto make the particle scattering colorants that are used in the presentinvention embodiment. For example, the primary particles can be eitherorganic, inorganic, or mixed organic and inorganic. Likewise, theoptional material that is dispersed within the array of primaryparticles in the particle scattering colorants can be organic,inorganic, or mixed organic and inorganic. In cases where the particlescattering colorants would be too opaque to optimize visual colorationeffects if only gas filled the void space between primary particles, itis useful to use either a liquid or solid material in such spaces. Suchliquid or solid material can minimize undesired scattering effects dueto fissures and grain boundaries that interrupt the periodic packing ofthe primary particles. In such case, it is preferable if such fluid orsolid has a refractive index in the visible range that is within 5% ofthe primary particles.

Another method for providing useful particle scattering colorantsutilizes polymer primary particles that form an ordered array in polymerhost, which serves as a binder. Films suitable for the preparation ofsuch particle scattering colorants were made by E. A. Kamenetzky et al.as part of work that is described in Science 263, pp. 207-210 (1994).These authors formed films of three-dimensionally ordered arrays ofcolloidal polystyrene spheres by the ultraviolet-induced setting of aacrylamid-methylene-bisacrylamide gel that contained an ordered array ofsuch spheres. The size of the polymer spheres was about 0.1 microns, andthe nearest neighbor separation of the spheres was comparable to thewavelength of visible light radiation. A method for producing filmsconsisting of three-dimensionally ordered polymer primary particles thatdo not utilize a binder polymer is described by G. H. Ma and T. Fukutomiin Macromolecules 25, 1870-1875 (1992). These authors obtained suchiridescent films by casting an aqueous solution of monodispersedpoly(4-vinylpyridine) microgel particles that are either 250 or 700 nmin diameter, and then evaporating the water at 60° C. These films weremechanically stabilized by a crosslinking reaction that used either adihalobutane or p-(chloromethyl)styrene. Particle scattering colorantssuitable for the present invention embodiments can be made by cuttingeither of the above described film types so as to provide particles ofdesired dimensions. One preferred cutting method is the process used byMeadowbrook Inventions in New Jersey to make glitter particles frommetallized films. Various mechanical grinding processes might be usedfor the same purpose, although it should be recognized that lowtemperatures might be usefully employed to provide brittleness thatenables such a grinding process. For use as particle scatteringcolorants, it is preferably that the cutting or grinding process produceparticles that are of convenient dimension for incorporation withoutsubstantial damage in the host matrix, which is preferably a polymer.

The particle scattering colorants of this invention embodiment arepreferably formed in required sizes during the aggregation of primaryparticles. Any methods used for post-formation reduction in particlesizes should be sufficiently mild as to not interfere with the desiredperiodicity of the primary particles. Likewise, processing conditionsduring commingling of the particle scattering colorant in either thepolymer matrix (or a precursor therefore) and other steps leading to theformation of the final article should not substantially destroy theoptical effect of the periodic assembly of primary particles. Forparticle scattering colorants that are not designed to be mechanicallyrobust, preferred processes for mixing of particle scattering colorantand the matrix polymer (or a precursor thereof) are in a low viscosityfluid state, such as in a monomer, a prepolymer, or a solution of thepolymer used for the matrix. For such polymers that are not designed tobe mechanically robust, film fabrication and article coating usingsolution deposition methods are preferred for obtaining the particlescattering colorant dispersed in the shaped matrix polymer. Likewise,for such non-robust particle scattering colorants, polymer matrixformation in shaped form by reaction of a liquid containing the particlescattering colorant is preferred, such as by thermal polymerization,photopolymerization, or polymerization using other actinic radiations.Reaction injection molding is especially preferred for obtaining moldedparts that incorporate particle scattering colorants that are notmechanically robust.

In another embodiment of this invention the particle scattering colorantconsists of primary particles that are translationally periodic in twodimensions, rather than in three dimensions. Fiber-like primaryparticles having an approximately uniform cross-section orthogonal tothe fiber-axis direction tend to aggregate in this way when dispersed insuitable liquids. Likewise, spherical primary particles tend toaggregate as arrays having two-dimensional periodicity when deposited onplanar surfaces. For example, such particles can be formed on thesurface of a liquid (or a rotating drum) in a polymer binder thatadhesively binds the spherical particles into two-dimensional arrays.These array sheets can then be either cut or ground into the particlesizes that are desired for the particle scattering colorant.

For all of the above invention embodiment of particle scatteringcolorants that consist of translationally periodic primary particles, itis preferable for the volume occupied by the particle scatteringcolorants to be less than about 75% or the total volume of the matrixpolymer and the particle scattering colorant. The reason for thispreference is that the use of low loading levels of the particlescattering colorant can lead to improved mechanical properties for thecomposite, relative to those obtained at high loading levels. Asdescribed above for particle scattering colorants that are notaggregates of periodically arranged primary particles, the visual effectof the particle scattering colorants consisting of ordered arrays ofprimary particles can be enhanced using electronic transition colorants.Such means of enhancement, as well as methods for achieving color changeeffects that are switchable, are analogous to those described herein forother types of particle scattering colorants.

From a viewpoint of achieving coloration effects for polymer articlesthat are easily eliminated during polymer recycling, particle scatteringcolorants that consist of translationally-ordered primary particlearrays can provide special advantages, especially if the primaryparticles do not substantially absorb in the visible region and thepolymer article does not include an electronic transition colorant. Thereason is that processing steps that disrupt such arrays can greatlyreduce coloration effects. From this viewpoint of polymer recycling, itis useful to provide particle scattering colorants that are convenientlydisrupted by either thermal, mechanical, or chemical steps.

Various applications for which the compositions of this invention haveutility will be obvious to those skilled in the art. However, for sucharticles having switchable coloration or switchable transparency that isbroadband in the visible, more detailed descriptions of applicationsembodiments are provided in the following. One such application is inprivacy panels, windows, displays, and signs in which theelectric-field-induced switchability of the refractive index of aparticle scattering colorant, an electronic transition colorant, or oneor more matrix components provides either device operation or anenhancement of device operation. In one example type, theelectric-field-induced-change in the refractive index difference betweenparticle scattering colorant and the surrounding matrix component can beused to change either (1) the transparency of an overcoating layer on asign (so that an underlying message is switched between visible andinvisible states) or (2) the transparency of either a privacy panel or awindow. For displays and signs, an electric field applied to a matrixlayer containing the particle scattering colorant can cause the degreeof particle scattering to change—therefore changing the effectiveviewing angle for an underlying message (such as produced by aback-lighted liquid-crystal display or other types of static orchangeable information-providing materials). The electric field canprovide switchable properties to either the particle scatteringcolorant, the matrix material for that colorant, an electronictransition colorant, or any other kind on information display material,or any combination of these materials.

Most preferably, the direction of a refractive index change provided bya particle scattering colorant (caused by an ambient influence, such asan applied electric field, temperature, time-temperature exposure,humidity, or a chemical agent) is in an opposite direction to that ofthe host materials. In this preferred case, the sensitivity of particlescattering to applied electric field or other ambient influence isenhanced by the refractive index change of both the particle scatteringcolorant and the matrix material for this colorant. Most preferably,such difference in the direction of refractive index change for particlescattering colorant and matrix material is for all possible lightpolarization directions. For the above applications, electric fields canbe applied in either patterned or unpatterned ways and differentelectric field can be applied to the particle scattering colorant andother materials, such as the electronic transition colorant. In generalthe local field that is across a particle scattering colorant in amatrix depends upon the state of aggregation of that colorant in thematrix, so a patterned variation in such degree of aggregation can beused to provide a patterned difference in the response of the particlescattering colorant to an applied electric field. For example, if theelectric conductivity and the dielectric constant of the particlescattering colorant are both larger than that of the matrix, anincreased voltage drop across the particle scattering colorant can beprovided by increasing the degree of particle aggregation. If theswitchability in particle scattering is largely a result of the electricfield influence on the particle scattering colorant, such aggregationcan increase the switchability.

Display or lighting devices that involve electroluminescent compositionsprovide special application opportunities. For example, theelectric-field-switching of particle scattering can be used to eitherchange the degree of diffuse light scattering from electroluminescentlight source or to provide a patterned distribution of light emission.In a preferred case, particles of the electroluminescent compositionserve as a particle scattering colorant. Another application of thisinvention in the lighting area is for light bulbs and lighting fixturesthat slowly become transmissive after the light switch is pulled, whichis an application of thermochromic materials that is described in U.S.Pat. No. 5,083,251, which is incorporated herein by reference. Suchlight sources are sought after to provide natural time-dependentlighting effects reminiscent of the rising of the sun. An example ofsuch technology that uses a vanishing Δn embodiment is obtained byemploying a particle scattering colorant that at room temperature has arefractive index that is unmatched at any point in the visible with thatof the host matrix. The particle scattering colorant is selected so thatthe heating of the light source causes the refractive index or theparticle scattering colorant and the matrix to become matched in thevisible. Hence, the heating process eliminates particle scattering atthe matching wavelength, so that the light source becomes moretransmissive. If this matching is desired to be broadband, then thereshould be little dispersion of the refractive index difference betweenthe particle scattering colorant over the visible wavelength range. Insuch case where little dispersion in Δn is wanted, the particlescattering colorant and the matrix can be chosen so that the differencein n_(F)−n_(C) for the particle scattering colorant and the matrixcomponent is smaller in absolute magnitude than 0.0001. For thisapplication mode, it is most preferable if the match between therefractive index of the matrix and that of the particle scatteringcolorant is achieved discontinuously upon increasing temperature above adesired temperature as a consequence of a discontinuous phase transitionof either the particle scattering colorant or the matrix material.Otherwise, the color of the transmitted light will vary somewhatcontinuously with the temperature of the particle scattering colorantand associated matrix material.

Indicators devices for chemical agents, pressure, temperature, moisturepickup, temperature limits (such as freeze or defrost indicators), andtime-temperature exposure provide other applications opportunities forthe particle scattering colorants of this invention. For such devices,either reversibly or irreversibly switched coloration can result as aconsequence of switchability in either the refractive index or theelectronic transitions of either particle scattering colorants, matrixcomponents, or electronic transition colorants. For the mentionedtime-temperature indicators, a color change can indicate that either adesired thermal exposure has occurred (such as for product processing)or that an undesired thermal exposure has occurred (leading to undesireddegradation of a perishable product). Using the vanishing Δn embodiment,the wavelength at which a match in refractive index occurs betweenmatrix and particle scattering colorant can be a function of integratedthermal exposure. For example, polymer films that are used for thepackaging of frozen vegetables can undergo a color change when thevegetables have been suitably heated for consumption. As another exampleof the use of the vanishing Δn embodiment for indicating successfulprocessing, a resin that is undergoing set (such as circuit board) cancontain a particle scattering colorant in the setting matrix. Thechanging refractive index difference between the particle scatteringcolorant and the matrix then provides a color response that indicateswhen satisfactory resin set has occurred. A similar useful example ofthe application of particle scattering coloration (in the vanishing Δnembodiment) is for the indication of moisture pickup for polymers, suchas nylon 6—so as to avoid the unsuccessful processing that would occurif the polymer has too high a moisture pickup.

Particle scattering colorants of this invention also enable theconvenient labeling of articles, such as polymer films, using thethermal or photochemical changes in refractive index or electronictransitions that occur as a result of patterned laser beam exposureduring high speed product packaging operations. For example, numberswritten by a laser on a polymer film used for packaging can becomevisible as a result of light-induced changes in the refractive indexdifference between the particle scattering colorant and the matrix. Suchswitchable particle scattering colorant/matrix combinations can also beused as a signature in order to thwart product counterfeitingactivities. The particle scattering colorants can even be used forapplications where the switchability in refractive index match atultraviolet light wavelengths provides materials operation. For example,an intelligent sunscreen for bathers can be provided by dispersing aparticle scattering colorant in a fluid matrix that is initially matchedat ultraviolet solar wavelengths with that of the particles. Alight-induced change in refractive index of either the matrix or theparticle scattering colorant (so that refractive index matching nolonger occurs) can provide an enhanced effectiveness of the sun screenas a function of increasing solar exposure.

The particle scattering colorant embodiments of the present inventionare especially useful for the polymer articles formed by desk-topmanufacturing methods. The prior art technologies for desk topmanufacturing (which is also called rapid prototyping) are described inModem Plastics, August 1990, pp. 40-43 and in CHEMECH, October 1990, pp.615-619. Examples of such methods are various stereolithographytechnologies that involve either the patterned electron beampolymerization or patterned photopolymerization of monomers. In suchcase the particle scattering colorant and optional electronic transitioncolorant can be dispersed in the photomonomer-containing fluid. Inaddition to providing coloration, such materials can provide additionalbenefits of reducing shrinkage during resin cure. Vinyl ether oligomersand monomers, that are used in conjunction with triarylsulfonium salts,are especially preferred for these applications. This is theultraviolet-cured Vectomer.™.system that has been developed by Honeywell(Morristown, N.J., formerly AlliedSignal). The particle size of theparticle scattering colorant, as well as other possible solid additives,should be sufficiently small that these particles do not settleappreciably during the fabrication of an article. For this reason,particle scattering colorants that have colloidal dimensions areparticularly preferred. Another method for rapid prototyping is theLaminated Object Lamination Method in which roll-fed sheets of polymerare cut by a soft-ware guided light beam—thereby building up the articleone sheet at a time. In this method the particle scattering colorant andoptional electronic colorant can be either located in the polymersheets, the adhesive that is used to bind the sheets, or both. Inanother method used for rapid prototyping, thin layers of a powder aredeposited on the surface of the article being constructed, and theselayers are fused in a patterned manner using a light beam.Alternatively, a binder (or a precursor thereof) is sprayed in apatterned manner on the powder (such as by using ink-jet spraying),thereby enabling article shaping in three dimensions. As anotheralternative, the powder layers can be replaced by a squeegeed gel layerthat is photoset in a patterned manner. In these methods, the particlescattering colorants and optional electronic transition colorants ofpresently described invention embodiments can be incorporated in theinitial powders, the binder, the gel polymer, or combinations thereof.Another technology for rapid prototyping builds three-dimensionalarticles by the patterned extrusion of thin coils of polymer. In suchcase, the particle scattering colorants and optional electroniccolorants of the present invention can be additives to the moltenpolymer. In any of the above described technologies for rapidprototyping, material coloration can be obtained by using either thelarge Δn embodiment or the vanishing Δn embodiment of the presentinvention.

Accordingly, it can be seen that the present invention provides anapparatus and method for controlling the application of a coating to asubstrate, where particles in the coating are selected based onelectric, magnetic, or photo/light properties, and are dispersed in thecoating to provide a desired physical color. In one approach, theapplication of the coating to the substrate is controlled using anelectric or magnetic field. In another approach, a pattern is formed ina coating on a substrate by targeting an electric, magnetic or photofield to specific locations on the coating. In another approach, thispattern or code is scrambled by targeting an electric, magnetic or photofield to specific locations on the pattern or code. In yet anotherapproach, the color of a coating or the color of the pattern within thecoating is shifted to match the color of the substrate so that thecoating or the pattern within the coating appears to be erased. Thecolor shift can be achieved by applying an electric, magnetic or photofield.

Note that in the embodiments described herein, different coatings havingdifferent electric, magnetic and/or photo properties may be usedtogether. For example, a first coating may be used that provides aphysical color change when an electric field is applied, while a secondcoating is used that provides a physical color change when a photo fieldis applied. In this case, separate equipment for generating the electricand photo fields can be used together.

While the invention has been described and illustrated in connectionwith preferred embodiments, many variations and modifications as will beevident to those skilled in this art may be made without departing fromthe spirit and scope of the invention, and the invention is thus not tobe limited to the precise details of methodology or construction setforth above as such variations and modification are intended to beincluded within the scope of the invention.

The following specific examples are presented to more particularlyillustrate the invention, and should not be construed as beinglimitations on the scope of the invention.

EXAMPLE 1

This example shows how a nanoscale silica can be used as a particlescattering colorant in a coating.

A nanoscale silica, Snowtex Z-L, was diluted {fraction (1/200)} withwater. The resulting solution was put onto a black-bottom watch glass. Alight blue color resulted.

EXAMPLE 2

This example shows how a nanoscale zinc oxide can be used as a particlescattering colorant in a coating.

A dispersion of zinc oxide (Nyacol DP5370) was diluted in water (10 ulzinc oxide solution/190 ul water). The resulting solution was put onto ablack-bottom watch glass. A periwinkle blue solution resulted.

EXAMPLE 3

This example shows how zirconia can be used as a particle scatteringcolorant in a coating.

A dispersion of zirconia in water was diluted with a refractive indexsolvent of 1.53 (620 mg of zirconia solution/1.8 g total). The resultingsolution was put onto a black-bottom watch glass. A light blue color wasevident in the water layer.

1. A coating composition for application to a substrate, comprising:fine particles that comprise onion skin particles wherein at least onelayer comprises a particle scattering colorant and at least one particlecomprises at least one material having electric properties, magneticproperties, photo properties or a combination thereof; and a matrixcomponent; wherein application of an electric field, a magnetic field, aphoto field or a combination thereof to the fine particles causeagglomeration of the fine particles in the matrix and a concomitant atleast one color shift, color reduction, color loss, pattern scrambling,or pattern loss.